These Kaiju on Pacific Rim are actually hard to distinguish their true colors. You look at Otachi, and you think she's blue. Look at concept art and stuff, and she's grayish-blue with yellowish-white on the underside. You look at Mutavore and you think that he's grey. But when you look at it closely, you can't decide wether he's brown, yellow, or purple. Especially his glowing essence.
Actually, Mutavore is a hard case for me to color. I know he's gray in color. But if you look closely at concept art, pictures, and all that, you can notice that his claws, head, and she'll are some sort of yellowish brown color and his body is purplish blue. Then, as a second layer of color, he is given this grayish tone to avoid any resemblance to Hardship.