RGB vs CMYK: what it is and what it is for

1What are RGB and CMYK?

They are two ways of describing a color with numbers. RGB stands for red, green, blue and works by adding light: a screen starts from black and turns on three kinds of light-emitting dots; combined at full strength they give white. CMYK stands for cyan, magenta, yellow and black (the K comes from key) and works by subtracting light: it starts from white paper and each ink absorbs part of what the paper would reflect.

Each model is the natural language of its medium. Your camera, your phone, a monitor and almost every image on the internet use RGB. A print shop working with inks uses CMYK, and many office or photo printers do that translation internally for you.

2Why do colors change when printed?

Because each model has a gamut, meaning a set of colors it can reproduce, and the CMYK gamut is smaller than RGB in several areas, especially intense blues, bright greens and very saturated oranges. When a screen color falls outside what the inks can achieve, it is replaced by the closest possible one, and the result looks duller.

On top of that, a screen emits light while paper only reflects the light around it. A screen white is a light source; paper white depends on the lighting and the type of paper. That is why a color can look different depending on whether the paper is glossy or matte.

3Converting between models and color profiles

Converting RGB to CMYK is not a simple fixed formula: it depends on a color profile that describes how a specific combination of inks, paper and press behaves. That is why print shops tell you which profile they prefer, and the same file can come out slightly different depending on the one applied.

There is an important detail: in practice the conversion is one-way. If you convert an image to CMYK and then back to RGB, the out-of-gamut colors that were clipped do not come back. That is why you should always keep the RGB original and convert a copy only when the print shop asks for it.

• Screen, web and social media: RGB.
• Professional printing: ask first what format and profile is needed.
• Office printer: usually you can just send RGB and let the driver convert.
• Grayscale: a separate case, with a single lightness channel.

4Myths and common mistakes

A frequent myth is that CMYK is better than RGB. It isn't: it is different, and better only for printing with inks. Another mistake is converting everything to CMYK just in case; if your file will be viewed on screen, converting it only takes away vibrancy.

It is also common to use RGB black (0, 0, 0) for text that will later be printed at a print shop, which can result in a black built from mixed inks instead of pure black ink. In small text that creates blurry edges. And trusting how a color looks on your uncalibrated monitor is a constant source of surprises.

5Common problems and how to solve them

If the printed color comes out different from what you saw, the first step is to ask for a printed proof before a large print run. If you work from a PDF, check which color space that file uses; a PDF can mix RGB and CMYK images on the same page.

If you need a light file without color, converting to grayscale is a different and predictable route. And if the PDF is too heavy to send to print or by email, reducing its size with compression can change the quality of the images, so check the result before sending it.

If in doubt, a good habit is to ask the print shop before you start: which color model, which profile, which bleed and which delivery format they accept. Five minutes of asking saves reprints, and lets you design from the start with the final destination in mind.

6RGB, CMYK and Rayo PDF GO

Rayo PDF GO is built for everyday work with documents and photos: merge, split, compress and convert. It is not professional prepress software and does not offer CMYK color profile management; if your job requires a print-ready PDF with specific profiles, use design software and confirm the requirements with your provider.

What you can do is PDF to grayscale for black-and-white versions that use less ink, Compress PDF to lighten files and PDF to images or Images to PDF to move content between formats, always with processing on your device.