How do I calculate the color of a compound?
Since this is quite a broad question, I will also only be able to provide an answer in very broad terms.

The short answer is: You cannot. At least not with (a real) pen and (real) paper.

The slightly longer answer is:
With a simple pen and paper approach this is virtually impossible. Even the use of more elaborate methods like MO theory can lead you to completely wrong results. Electronic transitions involve excited states, which can be incredibly hard to calculate. In most cases you can get an approximate spectrum by using time-dependent density functional theory. This can be right for the wrong reasons - it is still somewhat a crude theory. The best results you will get with multi-reference methods or big automated computations, which may take a long time on a supercomputer.  

In other words: There is not easy, general, rigorous way to determine the colour of a compound.