When painting a solid color can there be matching issues when pieces are painted separately at different times?

Yes to all of the above !

When I did body work, white always gave our painters the biggest problem. You think white is just white but, most have small tints of yellow, blue or red that you don't even notice until you try to match the paint.

White is the presence of ALL colors. That is why it is so hard to match! differing quantities of any and it will change it. Non-metalic black is one of the easiest colors to match. (absence of all color)