I personally like to model first but most times I leave the model as several bigger parts. I usually do this because sometimes a bit could block my brush from reaching a part of a model propperly. For example if I were to put an arm to a models body it might block my brush from properly reaching a bit of the models chest so I paint the arm seperately and glue it on once the part it would block and the arm itself are painted. Its different model to model so play around with what fits you.
Idk for the colours but as to when to paint it:
I personally like to model first but most times I leave the model as several bigger parts. I usually do this because sometimes a bit could block my brush from reaching a part of a model propperly. For example if I were to put an arm to a models body it might block my brush from properly reaching a bit of the models chest so I paint the arm seperately and glue it on once the part it would block and the arm itself are painted. Its different model to model so play around with what fits you.