Spike notices hair. People who have tattoos or piercings notice them on other people, those with Mini Coopers wave to each other on the road, at least they do in America. Everyone looks out for those people that are part of their particular tribe of affectation. For Spike, it's hair.
He's two songs into his first set when he sees the guy at the bar. Well, when he sees his hair. From the back, the man is nothing special - his clothes are kind of worn and they don't fit very well. He looks to be tall and kind of broad - just a normal bar patron. But his hair - his hair is something special. Longer than the collar of the faded plaid shirt, and it's wavy. Not curly like Spike's own would be if he let it happen, but actual waves - the kind that catch the light and reflect it back. The hair is dark, too, and Spike's been a sucker for dark hair forever.
He's been at this singing thing for a while, and he can do his sets without thought. He knows just when to lean intimately into the microphone, the exact timing for tossing his head back to hit a higher note. It's easy, and it lets him watch the motion of that glorious hair as the man at the bar drinks deeply from his glass of whiskey. That's when he recognizes Xander. And stumbles slightly over the words to Paint It Black.
He spends the rest of the song thinking. Thinking about Xander and wondering how he'd wound up here and, more to the point, why. It has to be an accident - they'd parted badly, with yelling and tears. Well, Spike had yelled, and he'd been the one with the tears - so very, very angry about Buffy's resurrection, and so very, very hurt that Xander hadn't told him. He watches closely, and when Xander turns toward the door, he studies the revealed profile. His face is pale and drawn; the eye that he can see is shadowed. The hand that comes up to run through that vibrant hair trembles. And in that instant, Spike forgives. And when he introduces his next song, he says that it's for his friend.