This might help you.

From that page:
By a hexadecimal triplet or quad in the form "#RRGGBB" and "#AARRGGBB" respectively. For example, the color red corresponds to a triplet of "#FF0000" and a slightly transparent blue to a quad of "#800000FF"
is what you would be in interested.
More precisely this: "#AARRGGBB"

AA - alpha (opacity)
RR - red
GG - green
BB - blue

Hope it helped.