To gauge the value (relative lightness/darkness of a color), I hold the model under a strong light (Sunlight also works). For a white area, I start to add black to the white very slowly until I reach the value of the shadows I see under the light. Using a generic "gray" can lead to a LOT of contrast. When the contrast is high, people will see that you did a wash, and it may distract attention from the build. I'm a FIRM believer of keeping effects (washes, drybrushes, etc.) subtle. The generic gray may be easier & take less time, but I feel it is less convincing. (I even apply different color washes to different areas of camouflage, and the diffrence between this way & a gray or black wash is dramatic)
The above is if you are trying to replicate shadows. Hope this helps.