Having played Omega Ruby, I can confirm Primal Groudon's skin to have a similar metallic shine to it as Primal Kyogre and Mega Rayquaza's bodies. I personally think the colors on the Primal Groudon sprite are really off; way too pale and dark. The red is much brighter on Primal than regular Groudon and more of a magenta-red than red-red, to put it very basically.The one thing I'm worried about, is that if we should keep his skin that purple hue. The box art and official art work seem to make it a purple like it is now. Pokemon Channel/Dream World/Center art have it as a dark almost navy blue. In game Primal Kyogre is a metallic more shiny color of its normal coloring.
Mega Ray is metallic too, not sure if that will change anything. I know nothing about P Groudon, Sapphire Fan for life B) I'll look at the models more when I play again.
Additionally, the tail retains its overall thickness instead of becoming thinner nearing the tip, is much longer on Primal and the black spikes turn longer the more they progress to the back.