Color on color underbased have been used on occasions but it's rare. That purpose is for unique effects. Imagine the same done with white ink on white. It softens up a limited colors print by using Wet on wet. The only time to use this technique is when you don't need to pop the colors off the shirt. Let's say for tone on tone prints.
If you intend for your top colors to be bright on dark shirts, you need a bright underbase. Then, a flash after...to make sure that base stays bright and does not get picked up.