Changing winds

A piece of Mars: There are two sets of ripples here: tan ones and gray ones, each oriented to a different wind (scene is 300×225 m, or 984×738 ft). The gray ones sit on top of the tan ones, so the gray ones are younger. Now come the fun questions: why the different colors? Are they made out of different material (and if so, why), or are the older tan ones different because the gray sediment has weathered to tan over time? (HiRISE PSP_002387_1985, NASA/JPL/Univ. of Arizona)