First thing first, design it's the opposite of codding. Codding (as far as I know) is like math : 1+1=2 and there is no other way around. In design, for one project, there are plenty of way to make it.
You need to learn the basis of the colors, what there mean, how to use it together, witch colors goes together and witch does not. Do the same with de shapes. Then tries to analyse good design and why they are good. A bit of art history is good too if you want to go further.
After that, it's mostly experience and feeling.