Silverado 4500/5500 coming next year.

Camo is used to hide some of the details and it also avoids the issue of having your soon to be released vehicle showing dirt, scratches, dents, etc. The test mules get a lot of abuse and the manufacturers don't want the first memorable images to clearly show the "war wounds" suffered during the test mule phase. It also avoids the need to use care in finishing and painting the test mule panels.