Can you regard the color of a new suit that you'll buy as a decision?

Yes and NO.

I would consider the color as preference. but in some cases color would be a decision

e.g in my brother wedding , a white suit was really a decision he made

i would consider buying a suite as a decision.  but a colour based on prefrence.

Definetly this will be decision because the colour of the dress represents your personality. A Project Manager's personality alongwith with the abilities makes him prominent among the team members.

Since there are multiple factors (where would the suit be worn, how often will the suit be worn) that may be relevant in choosing your next suit, I would certainly think that the color of your next suit is a decision.

As an example, I would not buy a pink colored suit and wear it to work.