RGB vs CMYK: which one do you actually need?

The short answer

Use RGB for anything that will be looked at on a screen. That is websites, social posts, email, slides, video, and every image on this site.

Use CMYK only when a commercial printing press is involved and the printer has asked you for it. That condition matters more than it sounds. Home printers, office printers and most online print services take an RGB file and convert it themselves, using a profile built for their own machine and paper. Converting first, by guessing, replaces their good conversion with your worse one.

Light against ink

A screen is dark until you turn it on. Every pixel has a red, a green and a blue element, and color is made by adding light: more of all three gives you white, none of them leaves you with black. This is why a monitor in a dark room can look startlingly vivid. It is producing light rather than reflecting it.

Paper is the opposite. It starts white, and it is white because it reflects nearly all the light hitting it. Ink works by subtracting: cyan ink absorbs red light and reflects the rest, magenta absorbs green, yellow absorbs blue. Lay down all three and in theory you absorb everything and get black.

In practice you get a muddy brown, because real ink is not perfect. So printing adds a fourth, actual black, which is the K in CMYK. It stands for key, the plate the others were aligned to, though remembering it as blacK does no harm.

Why print always looks duller

Because ink cannot reach everywhere light can. The set of colors a process can physically produce is its gamut, and the CMYK gamut is smaller than the RGB one nearly everywhere that matters.

The worst losses are consistent and worth knowing before you design anything for print:

• Saturated blues go purple or gray. This is the one people notice most, and it ruins more brand colors than anything else on the list.
• Bright greens flatten toward olive.
• Neon and fluorescent anything simply does not exist in four-color printing. Those are printed with special inks, ordered separately, at extra cost.
• Vivid oranges and reds lose their glow, though less badly than blues.

When a file is converted, every color outside the ink range is replaced by the nearest one inside it. Nothing warns you. The picture still looks like the picture, it just sits a little flatter, and two colors that were clearly different on screen can arrive identical on paper.

Why no browser tool can do this

This site converts between a dozen image formats in your browser and will not offer you an RGB to CMYK conversion, so it is worth saying plainly why.

A browser works in RGB. The canvas that every tool here draws on stores four numbers per pixel, three of them red, green and blue, and there is no CMYK equivalent to write to. A browser tool that claims to convert is doing one of two things: showing you a simulation of how the colors might print while handing back an ordinary RGB file, or uploading your image to a server that has real software on it.

Neither is what you wanted. The first is a preview wearing the wrong label, and the second means your file left your machine.

There is also a deeper reason, which is that there is no single correct conversion. The right numbers depend on which inks, which press and which paper, because ink soaks into uncoated stock differently than it sits on coated stock. That information arrives as a color profile, and printers supply theirs precisely because guessing produces the wrong answer. A tool that converted without asking which press would be inventing the part that matters.

What to do instead

Almost always: send RGB, and let whoever is printing convert it with their own profile. Beyond that, the useful work is done before the conversion rather than during it.

Find out what colors you are actually working with. If a brand color is a saturated blue, you will lose it in print and you should know that at the design stage rather than at the proof stage. The color extractor reads the dominant colors out of an image and gives you their values, which is the fastest way to see whether anything in your artwork is sitting in the danger zone.

Pull the risky colors back yourself. A color you desaturate deliberately looks considered. The same color desaturated by a conversion you did not watch looks like a mistake. The color changer lets you shift hue and saturation and see the result, so you can make that call rather than have it made for you.

Get the file itself right. Printers usually want a high-resolution PNG, TIFF or PDF rather than a JPG, and at a size that suits the finished piece. The image converter and the resizer handle both, in your browser, without the file going anywhere.

Sending a file to a printer

The short version, in the order it usually matters:

1. Ask what they want. Every printer has a specification and most publish it. It will name a format, a resolution and, if they want CMYK, the exact profile.
2. Send RGB unless they said otherwise. Their conversion is better than yours because it is built for their press.
3. Send it at print resolution. Roughly 300 pixels per inch of finished size, so a 6 by 4 inch print wants about 1800 by 1200 pixels. Enlarging a small file does not add detail.
4. Keep your original. Whatever you send, do not overwrite the file you started with.

If the printer does ask for CMYK and supplies a profile, that is the point at which you need real design software. Photoshop, Affinity Photo and the free GIMP all do it properly, and all of them will apply the profile the printer gave you rather than a generic one.

Frequently asked questions

What is the difference between RGB and CMYK?

RGB is made of light and CMYK is made of ink. A screen starts black and adds red, green and blue light to build a color, so all three at full strength give white. Paper starts white and ink subtracts light from it, so cyan, magenta and yellow together give a muddy near-black, which is why a fourth ink, black, is added. They are opposite processes, and that is the whole reason the same color can look different in each.

Should I use RGB or CMYK?

RGB for anything that will be looked at on a screen, which is almost everything: websites, social posts, email, presentations, video. CMYK only when a commercial printing press is involved and the printer has asked for it. Home and office printers take RGB files and do the conversion themselves, so converting first usually makes the result worse rather than better.

Why do my colors look duller when printed?

Because ink cannot reach some of the colors a screen can. A monitor emits light and can produce a saturated blue or a vivid green that no combination of four inks on paper can match. Those colors are outside what is called the CMYK gamut, and when a file is converted each one is replaced by the nearest printable color. Bright blues, neon greens and vivid oranges lose the most.

Can I convert RGB to CMYK online for free?

Not in a browser, and be careful of any tool that says otherwise. Browsers work in RGB and cannot write a real CMYK file, so a tool claiming to convert one is either simulating the appearance and handing back an RGB file, or uploading your image to a server. A genuine conversion needs a color profile for the specific press and paper, which is why printers supply one rather than leaving you to guess.

Does converting to CMYK reduce quality?

It does not soften or compress the image, but it does discard color. The conversion is one way in the sense that matters: colors outside the ink range are replaced by the nearest printable ones, and converting back to RGB afterwards does not bring them back. Keep the RGB original and convert a copy.

What does CMYK stand for?

Cyan, magenta, yellow and key. Key is the black plate, called that because the other three were historically aligned to it during printing. It is not shorthand for blacK, though that is the easier way to remember it.