According to the key on the graphic the colors represent areas where incomes are either above or below the "median income". For instance GRAY represents areas where the income is from 75% to 125% of the median. Example: If median income in an area was $50,000 then half the residents of the area earned at least $37,500 but not more than $62,500.
As you see the GIF move through time more Green AND Pink/Red-ish areas emerge and crowd out the Gray.
The Green areas are where incomes are significantly greater than the median (getting richer--pulling away from the median upward) and the Pinkish to Red areas are where incomes are significantly less than the median (getting poorer---pulling away from the median downward).
Here are still shots I took of the GIF is you need to look at it at a more leisurely pace.