He'd seen her coloring change, and had realized on some level that she presented a more human look said....something. But it could just more convenient in Trench, which was almost entirely populated by humans. And she's alluded to her people being killed for being different, which he empathizes and sympathized with -

He was beginning to realize just how deep that fear of being regarded as other might go for her.

Keep it light, he told himself.

"I thought it suited you," he replies, matter of factly. "Sabine, her hair is brown naturally, but dyes it all the time. She'll get bored or well, be in some sort of mood, and switch to something else."