You have done well thus far. Let us move away from codes and ciphers for a moment.
A traditional riddle, no more and no less.
Only one colour, but not one size,
Stuck at the bottom, yet easily flies,
Present in sun, but not in rain,
Doing no harm, and feeling no pain.
The answer may be, quite literally, darker than you think.
You know where to put it when you are done.