I have a sudden urge to paint blue my computer to match the keyboard and the mouse and also the CPU. There is a type of an ink that I could use for it?
A great option would be the use of plasti dip. Plasti dip is a rubber-ish spray that is non conductive (since you mention the cpu you might as well want to paint your motherboard? If you mean the heatsink on top of the cpu, you'd need special paint that is thermal conductive).
Since it's rubber-ish, you can also remove it later on if you decide you don't like it (anymore) or want to sell some computer parts.
It's also fairly cheap and can be found in most car shops.
Linus, from Linus Tech Tips, recently did a video about this one. You could check it out here.
That depends on the material and the paint already on it. For plastics, I'd use paint made for scale modelling (Revell, Humbrol etc.). For metal, check your local paint store.
Don't use a spray can, it's impossible to prevent paint spray from getting everywhere in the computer.