RGB vs CMYK
What are RGB and CMYK in the first place?
Well, RGB and CMYK are colour spaces. Colour spaces are abstract models or systems describing colours as numbers. CMYK is used for printing and RGB is used for monitors, scanners and cameras.
So what is the difference between the two?
There are several ways to look at it, but technically, RGB is an additive colour model, where red, green and blue light (hence RGB) is added together, producing various colours. It starts with a black base (absence of light) and it adds (emits), you guessed it, light.
CMYK is a subtractive model, it starts with a white base (a sheet of paper, for example) and reduces the reflected light via ink. The four ink colours are cyan (greenish blue), magenta (purplish red), yellow and black. (The K in CMYK stands for key, not for black. The key plate is the one that contains the details and usually uses black ink (if there are only two colours and no black, it uses the darker colour), so it's still OK to think of the K in CMYK as of black.)
In short, RGB combines light to create brighter colours and CMYK combines inks to create darker colours.
Here's a practical example: everything you see on your computer monitor, tablet, and phone is in RGB mode, and they use a direct light source to display colour. (What you see on your TV is probably in YUV mode, though, but that is an additive space too.)
The Game of Thrones poster on your wall and that ticket for the Arcade Fire concert you printed on your home printer are in CMYK mode. (Your childhood drawings are not strictly CMYK, because you probably used more watercolours than four, but they are still subtractive.) They use paint/ink to affect the amount of white light reflected from the surface, thus producing a colour image.
Or, the SUN is RGB, and the MOON is CMYK. (This is not right, of course, but you get the point.) The sun is a direct light source, while the moon reflects light. That's the second way to think of RGB and CMYK.
And the third way is even easier. You can imagine CMYK as a limited version of RGB in terms of brightness (this is technically correct only if we talk about what you see on your monitor).
On the other hand, CMYK has a greater number of colour shades, at least in a perfect world. CMYK is a four channel mode (cyan, magenta, yellow and black), and with 8 bits (2^8 = 256 colours) per channel this means 256 x 256 x 256 x 256 = 4.3 billion colour shades. RGB only has three channels (red, green and blue), which means 256 x 256 x 256 = 16.7 million colour shades.
In other words, CMYK has a greater gamut (colour range), while RGB has a brighter gamut (none of the 4.3 billion shades would be so bright as RGB colours can be).
But you can't really see the full glory of those 4.3 billions on your laptop or on your t-shirt.
First, the human eye itself can distinguish "only" 10 million colours.
Second, everything you see on a monitor is, as said, in RGB mode, which can simulate CMYK, but is still limited to 16.7 million colours. Or to 900,000, really, because computers work mostly with sRGB (the colour space created for standard use on monitors and printers). And even if you use Adobe RGB in Photoshop, that still gives you about 1,3 million colour shades.
And third, you don't need so many colours. In Photoshop, for example, you can only enter integer percentages for the CMYK values, which gives you 100 x 100 x 100 x 100 = 100 million combinations out of the potential 4.3 billion. Because, let's face it, C:55,00% or C:55,04%, it doesn't make any difference.
And in practice graphic designers work with a lot less colours. To avoid differences between what the client sees on a monitor and what is printed (to make things worse, different printers give different results), designers often use colours swatches. The standard here is the Pantone matching system. The Pantone process colour swatch book has 3000 colours (although there are some swatch books that go up to 20,000+) and the Pantone solid colour swatch book has 1,114 colours.
And now that I've mentioned Pantone, I think I should say a few words about:
We've been talking only about CMYK printing so far, or, more correctly put, about four colour process printing.
A spot (or solid) colour, on the other hand, is one that is printed with its own ink, and not with a mixture of different inks. As mentioned, the most widely used spot colour system is Pantone.
Spot colour printing is practical when you are using few colours (no more than three), otherwise it gets expensive (because you need a new plate for each colour).
Also, CMYK can't recreate all Pantone colours. It is a subtractive model as well, but it uses 14 base colours, unlike CMYK's four. The CMYK counterparts are usually really close to the Pantone colours, but sometimes the difference can actually be noticed by the naked eye and you might want to use spot printing.
I won't go into any more details (I couldn't even if I wanted to, I've never had to use spot colours and printing so far), I just brought the subject up because I felt the post would be incomplete if I didn't. If you're interested, you can read more about spot (and global) colour here.
This completes the first part. Check out part 1.5 for an answer to the question What colour space should I work and save my files in?
Coming up: Graphic Design Basics: Colours Pt. 2 - Rich/True Black vs Default CMYK Black.