I think you can be free with the hair colour, while the consensus is that ALL Salamanders have Onyx skin and red glowing eyes, the lore seems to be silent on whether they continue to have hair after their transformation into Space Marine. The challenge I believe, is picking a hair colour that actually looks good with black skin. So far all pictures of Salamanders unhelmeted depict them as bald.
There is also the skin branding tradition that Salamanders have. For various reasons, they like to burn marks unto their skins to commemorate an important moment, including the head. Chapter Master Tu Shan whole head is supposedly covered with these scars hence no room for hair, presumably burnt off. In addition, they also burn themselves slightly before entering their power armour I think.
With all this burning practices, it may be that the average Salamander is bald not because he is really bald, its because of all the burning practices they have with their own bodies.
So Scounts with hair will still make sense I believe. You can even play around with their skin colour as the black carapace is not fully formed yet. As long as it is a "dark colour", should still be good. Just make sure they don't end up with pale skin like Raven Guard and your scouts should be good.
"The 6th Legion has a reputation." said Bear.
"All the Legions Astartes have reputations," replied Hawser.
"Not like ours," said Ogvai "We are known for our ferocity. We are thought to be feral and undisciplined. Even brother Legions consider us to be wild and bestial."
"And you're not ?" asked Hawser.
"If we need to be," said Ogvai. "but if that was our natural state, we'd all be dead by now."
He leaned down towards Hawser like a parent addressing a child.
"It takes a vast amount of self control to be this dangerous" he said.
Excerpt from "Prospero Burns" by Dan Abnett