Does your customer want "no swirls", or "no pearl"? If you mix PR or Alumilite Clear with mica powder, and pour well before the mix starts to gel, the mica flakes will tend to align before the resin sets and you won't have any swirls. The blank will, however, still have a sparkling reflective quality.
If you want solid color without any depth or pearlesence, you can use opaque pigments (such as carbon black and titanium dioxide) with clear resins, or you can use an opaque resin and tint it. It's easier to mix a color you like with clear, but you'll have to use a lot of pigment to make it opaque. On the other hand, one can readily turn Alumilite White black (or dark gray) with black dye. I haven't tried it, but you might also be able to lighten Alumilite Black with white dye. The trick is adding the right amount of pigment because the color doesn't show until the resin starts to cure. Some trial-and-error might be necessary.
I hope that helps,
Eric