HiRISE color can show mineralogical differences due to the near-infrared filter. The sources of channels on the north rim of Hale Crater show fresh blue, green, purple and light toned exposures under the the overlying reddish dust.
The causes and timing of activity in channels and gullies on Mars remains an active area of research. Geologists infer the timing of different events based on what are called “superposition relationships” between different landforms. Areas like this are a puzzle!