Nymphadora's hair is completely intriguing to him. He's found that if he looks very closely, it's never really the same shade twice. He laughs at this because it seems so Nymphadora-like to him, unique but so subtly so that you would really have to know the woman intimately before all facets of her personality were revealed.
Remus has learned that the happiness she shows to Molly is not the same as the one shown to Kingsley or himself. Just as her hair may be navy three days in a row but if he looks closely the one on Tuesday may have a more yellow undertone just as Thursday's may have a greener.
He loves this.
He could sit beside her for hours, examining each colourful strand for the differences from the day before and memorizing today's colour for comparison tomorrow. He wishes that he could spend his life merely studying this woman.
He sees this differentiation of colour as a silent way of expressing herself (even if she already does with everything else). Remus finds himself jealous of this, for he lives in constant fear of expression, even in the smallest ways as she does here.
He hopes that one day he might learn how to forget his fears, and truly live, but for now he will do so vicariously through his fascinating nymph.