The Raptor is a stealth aircraft and its paint plays a role in helping the plane remain avoid detection. It's an odd paint that leans into an almost pearlescent finish.
Because colors like Aggressor and Gunship greys are completely flat in finish, a lot of modelers are experimenting with different techniques to simulate the pearl iridescence of the Raptor's finish, including lightly misted coats of silver.
The link provided above demonstrates one of those experiments, where the builder used gunmetal mixed with various greys to simulate this effect using paints that are readily available in most local hobby shops.
Personally, I think the effect is pretty subtle at 1:1 scale and I can't say that this scales down well, but it's a matter of personal taste and how far one is willing to go in search of accuracy. Another example of art vs. science in this hobby.