In episode 16, timecode 9:10, of That Time I Got Reincarnated as a Slime, Milim refers to Rimuru's human form as being a "Silver-haired humanoid". Here is Rimuru (right) and Milim:
Did they just have a bad translation, or does "silver" refer to some broader range of hues in Japanese? Or maybe Milim has some sort of weird color blindness, or sees only in black and white? I'm pretty sure most watchers have assumed Rimuru's hair color is blue, the same color as his slime form.