Do designers use the same RAM (Radar Absorbing Material) coating across the length and breadth of a fighter aircraft?
According to this article and this book, they use a greater amount of wide-band structural RAM on the wing's edges, whereas they use a ceramic-matrix RAM on the exhaust nozzles. Not only is that the case, but the actual edges have proprietary material designed to be stronger, and have a different radar signature. Although, what you're seeing is probably a combination of the paint job as well as the material, considering the F-22's stealth is not only an aspect of radar, but also visibility. 

Most of the stealth of the F-22 comes from the physical shape of the F-22, and reducing its radar cross-section. There are also specialized materials that assist in obscuring its infrared signature. The topcoat, a type of paint, assists in reducing the infrared signature, especially.