From Memory Alpha:
"A yellow painted model of the Constellation-class NCC-7100 appears in Captain Picard's Ready Room aboard the USS Enterprise-D. This model is often mistakenly assumed to be of Stargazer... the model existed before the idea of the Stargazer, and was eventually used to create the filming model."
"In 1994, Galoob produced a toy model of the Stargazer as part of their Micromachines line. Interestingly enough, the coloring on the model was completely wrong and was actually based on the coloring of the NCC-7100 model seen in Picard's office."
Perhaps Picard, being busy with the demands of Captaining the Fleet's Flagship, simply hasn't gotten around to painting it yet? Geordi built a model of an ancient sailing vessel, and mentions that it took him two-weeks' worth of off-duty time. (I think Data or Picard comments that "this is what's been keeping you so busy.") The model was a gift for his old commanding officer.
Rick Sternbach in a post at the TrekBBS said that the model was indeed the Stargazer as she appeared just after the warp field stress test with the hull painted yellow to detect cracks in the hull. And if you people don't know who Rick Sternbach is well then you have bigger problems than this wiki can help with.
You mean to say there are people who don't know who Rick Sternbach is? By the way, question was about the color of the model.