Yeah, my 95 Deville, brothers 99 and mom's 97 all have this problem. It seems to effect all E/K platform cars from 94 to 1999 on Deville and 93 to 2002 (eldo) or 2004 (Seville/STS).
It is, as has been pointed out, completely independant of color; although black may fail more quickly due to greater heat absorbsion eventually they all have this problem.
By my observations, what happens is that the soft dash skin pulls away from the ABS plastic substrate (the rigid form that makes the shape and support structure of the dash pad) and curls/mishapes at the edges around the defrost grill and the center channel speaker cover. Once this occurs, the grille has nothing to grab for retention. It is true that the "tabs" on the edges of the grille are really too small to be effective, especially once the edge-curling begins.
My solution to this was to remove the dash pad from the vehicle, remove the double sided tape that held the underside of the dash pad skin to the rigid plastic at the edges and then use a polyurethatne glue (Elmer's makes some that works great, Gorilla glue is very similar) and then used clamps and pieces of 1x2 to hold the edges down tightly overnight until the glue was cured.
Do be careful with this, the glue is very messy stuff and requires solvent cleanup (mineral spirits) while still wet, and once it's dry you'll never get it off finish surfaces. It does expand as it cures, the excess that bleeds out can be trimmed with an xacto knife or similar intrument when fully dry.
The advantage to this is that the polyurethane glue is about the only thing that will adhere long term to both the ABS plastic and the foam layer under the dash skin. ABS is notoriously difficult to glue to even to itself (I'm in the plumbing business and see this problem routinely on older ABS pipe) and anything less (i.e. super glue, silicone caulk, hot glue, etc.) just won't do the job, or at least not for long.
Once it's all dry and ready to go, reinstall the dash pad and pop in the grille. The one problem I've found after doing this repair is that the grille still rattles a bit, because the opening is still not 100% original shape - once it curls and peels, you'll never get it back to fully original, as the heat permanently mis-shapes the groove where it snaps in. It is about a 95% effective repair in that the grille stays snapped in, just not as tightly as it once did. I have since added some velcro underneath that has eliminated the rattles I was still getting. Of course your results may vary depending on how severe the problem was on your car before fixing it.
It is dissapointing to me that GM used this design for upwards of 10 years and never improved it despite the fact that it was an obvious problem in many vehicles after only a couple of years in service. Maybe now that GM seems to be vastly improving the interior designs with the latest generation product, this will not be a recurring issue.
In any case, I hope others here find this useful, as it has worked very well for me, and my dash now looks like it's in a brand new car, despite it being 10+ years old.