RGB vs CMYK explained for designers

RGB and CMYK are two ways of describing color. RGB is how screens make color; CMYK is how printing presses make it. Your design tool almost certainly works in RGB. A press needs CMYK. Somewhere between your design and the paper, the colors have to be translated, and knowing a little about that translation helps you get the result you expect.

RGB: color made of light

Every pixel on your screen has three tiny lights: red, green and blue. Each can be dimmed or brightened, usually on a scale from 0 to 255. All three at 255 make white; all three at 0 make black. A HEX code like #FF6B61 is the same RGB numbers written in a compact form. Because a screen emits light, RGB can show very bright, glowing colors.

RGB numbers on their own are not quite enough to define a color: the same numbers can look slightly different on two screens. A color profile, such as sRGB or Display P3, pins the numbers to real colors. sRGB is the long-standing standard for the web and is what most design tools use. Display P3 is a wider range found on many newer screens.

CMYK: color made of ink

A printing press puts down four inks: cyan, magenta, yellow and black (the K). Each is described as a percentage, from 0% (no ink) to 100% (full ink). Paper without ink is the white. The inks are printed as fine dot patterns, and your eye blends the dots into colors.

In theory, full cyan, magenta and yellow together make black. In practice the result is a muddy dark brown, uses a lot of ink, and is hard to keep sharp, which is why presses add a separate black ink. That black ink is the key to many print questions, from rich black to total ink coverage.

CMYK numbers also need a profile. The same 50% cyan looks different on glossy coated paper and on uncoated paper, and on different presses. Profiles such as GRACoL 2013 describe one of these printing conditions.

Side by side

What happens when you convert

Converting from RGB to CMYK means asking: for this screen color, which mix of the four inks on this paper looks closest? A conversion uses two profiles: the RGB profile of the source (for example sRGB) and the CMYK profile of the destination (for example GRACoL 2013).

Three things happen during a conversion:

1. Most colors map closely. Skin tones, earthy colors, pastels and most photos convert with little visible change.
2. Out-of-range colors are pulled in. Colors outside what the inks can print become the nearest printable color. This is where the “my colors got duller” feeling comes from. See Why colors look different in print.
3. The black ink is decided. For dark and neutral colors there are many possible ink mixes that look alike, from lots of cyan, magenta and yellow with a little black to mostly black with a little of the others. The choice affects how much ink goes on the paper.

That third point is what Safer for press is about. It converts like Convert to CMYK and then, in dark and neutral areas, shifts the mix toward black. Printers call this gray component replacement (GCR). The color looks similar, but the total ink is lower: in our business card example, the average ink went from 219% (plain CMYK) to 196% at Medium.

When to convert, and who should do it

• Your printer asked for CMYK: convert before sending. Here is how.
• Your printer accepts RGB: you can send RGB, but converting yourself lets you see the result first.
• Digital use only (web, social, slides): stay in RGB. CMYK is for print.
• Keep your RGB original. Convert a copy for printing; if you edit the design later, export and convert again.

How CMYKflow handles each color in a PDF

• RGB with an embedded profile (for example a photo with its own profile): converted from that profile.
• RGB without a profile: treated as sRGB.
• Already CMYK, with or without its own CMYK profile: Convert to CMYK keeps the values. Safer for press can still lower the ink in dark and neutral areas. In our tests this held for plain CMYK, for CMYK tagged with a coated-paper profile and for a PDF with a print output intent; the profile tag or output intent itself is not carried into the converted file.
• Destination: GRACoL 2013, with converted colors capped at 320% total ink.

Pure black (#000000) text and shapes came out as black ink only in our tests, which keeps small text crisp. Very dark greys are converted like any other color.