Sootfur is described as a light gray tom as seen below, and is notably darker compared to his brother, Rainwhisker.
His mom is Willowpelt, a light gray she-cat and his father is Whitestorm, a fully white tom. This could impact his genetics but seriously Warrior Cats doesn't care about that. Besides, he could still be dilute or not because of what Whitestorm is hiding under his white.
In his official appearances his color remains relatively the same. In the James Barry version he is noticeably darker when looked at in next to the new graphic novels. (See below)
The prophecies begin volume 3:
Allegiances (I'm ignoring the shading on his head, I dont believe it concerns his color):
This seems to differ in all of his appearances where he is changing between being a bluish gray to a brownish gray. Soot, however, is not blue tinted at all and follows the light gray description he carries.
This is very accurate to his website family tree icon!
It is also true soot can be black, and I think during his death scene in Twilight and vigil scene in Sunset this is what the authors referred to instead of his allegiances description. This causes a bit of a question on his color, even though it is dark during the battle he is specifically called black pelted. Also in the following book, Sunset, his body is called gray-black three times.
The warrior cats website acknowledges this diffence in description by having two differing sprites:
I feel like it is just a commonly accepted fact that he is a light gray tom, because in most fanarts I see he stays this light gray, and seldom is blue tinted or black. I think this is because a big thing about his color is the contrast between his brother who is darker. I am unsure where the idea he is a black tom came from besides his death scene and body...
Pick whatever color you want when drawing him, though! I love looking at ALL of people’s designs especially when he looks like a stupid little idiot, so the color doesn't matter much.