Fire Elemental wrote:
Fire elementals vary in appearance—they usually manifest as coiling serpentine forms made of smoke and flame, but some fire elementals take on shapes more akin to humans, demons, or other monsters in order to increase the terror of their sudden appearance. Features on a fire elemental’s body are made by darker bits of flame or patches of semi-stable smoke, ash, and cinders.
I think that typically they'll be orangish-red and yellow, just being older and stronger doesn't mean they're necessarily magnesium flares or welding torches, but specific features can be other shades; like their 'mouth' could be black like soot or smoke, or their eyes might be bright yellow, almost white. I don't think anyone will care if you give some tertiary or peripheral colors of 'hotness' like white or blue or something in the flickering flames around them, but if you say it looks black or bright white, they'll likely assume it's something else (or at least, not what it is), like a blackflame elemental or a positive flame elemental.