What do motherboard RAM slot colors mean?

It means that the colored pair is a dual channel set and that you should install RAM as a pair to take advantage of it.

You should install 2 of the same sticks as a matching pair on the same color slots and then another 2 that are the same in the remaining two slots.

Ideally you want to have all memory be identical in a system or else you will end up with some memory being potentially downclocked (or voltage/multiplier) to the lowest sticks specifications.

Different manufactures have in the past used different layout rules so you really need to read carefully to make sure you are installing them in the right position/order.

http://www.tomshardware.com/forum/254308-30-install-dual-channel

http://www.hardwaresecrets.com/article/Everything-You-Need-to-Know-About-the-Dual-Triple-and-Quad-Channel-Memory-Architectures/133/5

The colors are nicely answered by Enigma. As for:

But after I tried to install a third one it always throws me a blue screen of death. Is there an order how should I install RAM to the board?

The answer is Yes, there is an order, and the details are found in your motherboard manual, which nearly always has detailed instructions for what order the memory slots should be filled, and which configurations will work, so you can simply put it in once and have it work, rather than

I just put the 2 RAM in, and after a few tries it always worked.