Spraying acrylic over enamel is no big deal. Acrylic won't bother enamel at all. Spraying enamel over acrylic will work fine as long as the acrylic is thoroughly cured. About the only combination you need to eally be careful with is laquer over acrylic. In that case, even with well-cured acrylic, the first couple of coats of laquer should be very thin "Mist" coats.
Yellow and white are problem colors for me as well. The only way I can usually get them to spray properly is to thin them a lot and then reduce the pressure somewhat. This allows the needle to be pulled back further opening more of the nozzle. This, of course, means that even more coats are necessary for good coverage.