A composer doesn't necessarily sit down and decide to "make music". A composer often has some sort of goal in mind, and yes, they will indeed seek out the best instrument for the job. The criteria may very well even include such things as "how skilled am I on the instrument"; someone may just noodle around on a keyboard or a guitar for something that may end up on another instrument entirely, for reasons of familiarity, or because the instrument has useful characteristics. For instance, a piano is a very useful instrument to write for a symphony orchestra, because even though it may not sound like any instrument that will be in the final composition, it's pretty hard for one person to play two flutes and two trombones at the same time, whereas the piano makes it easy to play at least the same notes, so you can hear the harmony.

Similarly for painting; you're looking at a world of difference between watercolor and oil (just to pick two examples), and what you chose depends on the goals.

I'm not sure your metaphor was as useful as you'd like.

Those analogies don't work. Paintings use multiple colors, and many pieces of music use multiple instruments; but for most programs, it's far simpler to use just one language. And if one is going to develop a large program over the span of many years, it's good to think carefully about what language to use before diving in, as that decision is not easily changed.