Some expressions take multiple parameters, separated by comma. In this case rgb() requires three numbers.
You can enter these numbers directly into the event (so called "hard-coding"), or you can use variables, or get them from the array etc.
But you can't feed to rgb() expression a single string containing three numbers, it won't accept it.
choose() expression is a perfectly good way to do it, however if you have lots of colors, it may become too long and difficult to edit in the future.
It will be no more than 10 or 12, so it seems like a good approach. I'm a bit OCD about keeping the event sheets simple, partly due to apprehension because there's still so much I don't understand, I want to try to limit complexity and keep it in line with my abilities. I'm learning every time I work, so that will change over time.
Thanks again!