GMCs had a wide variety of color combinations in 1950 (many more than RPO combinations available for Chevy trucks in 1950).
This link shows the variety of combinations in 1949 (GMC).
http://www.oldgmctrucks.com/photos/1949_GMC_PaintSpecifications.jpg That Suburban might be a '50; it is not a '53.
The whitewalls were not a RPO but the cream wheels could have been. I think that I once saw that truck on the side of the road in VT and it is nice. The price will most likely rise much higher (it is now about $10,000).
Tim
