I have a theory that McMaster-Carr actually sells every material you could ever want, but that it has these items so finely categorized that you just can't find the bulk of the more unusual ones because you don't know what to search for. Sort of like dark matter.
For example, you can search through their entire listing of "Plastics" and not find any two color sheets. If, on the other hand, you search for "Blank Engraving Plates," you find a whole category of inexpensive two-tone plastic (ABS or Acrylic) sheets in a variety of sizes and color combinations, with or without adhesive backing:
They're meant to be engraved with a hot iron that holds metal character stamps, but doesn't that sound PERFECT for laser engraving? I got some of the 2"x4" black surface, white core ABS plates with adhesive backing (Item# 4071T161, ten pack for $8.07!) to cut/engrave durable and easily readable face-plates for some custom PC104 enclosures, which came out quite nicely given that I spent a total of 5 minutes playing with settings before cutting the lot of them:
The plates come with a protective plastic sheet over the colored surface. I found that if I removed it before cutting, the engraving came out very clean and white, but that hot gases from cutting marred the "micro-thin" black surface. I left the coating on when I cut my set, which kept the un-engraved surface nice, but resulted in some of the black coating getting melted down into the white engraved lines (plus it was a real pain to pick out the bits of coating from letters with closed loops).
I was in a hurry, and what I got is totally functional, but in hindsight, I bet I could have had it both ways if I removed the plastic coating and put something more easily vaporizable, like masking tape, over the sheets. I also could probably have tuned the laser settings a little more finely. Oh well, no regrets. The next time I want a fancy cover plate for a project I'm totally going to try out some of the other colors!