Well "that gray" is a very unspecific definition of a color.

And even official artwork may varry slightly in tone.
And relictors are basicly grey marines with black shoulders.
Just use the grey you like more. And depending on your painting skills you could try mixing colors or try a black wash to darken your grey to a shade you think fits better.
Or alternatively supply us with an actuall reference picture for what you are going for. Or is it one of those two or the picture in codex:
SM?