color - How to extract RGB arguments as a list

Although the following lines of code work fine they are very awkward.

My questions are below

a = RGBColor[1, 0, 0]
b = RGBColor[0, 1, 0]

Is there a more compact way to construct ab?

Can the three entries of RGBColor[u,v,w] be extracted as a list?

Can the RGBColor argument be entered as a list RGB[{list}]?

ab = RGBColor[a[[1]] + b[[1]], a[[2]] + b[[2]], a[[3]] + b[[3]]]
Graphics[{{a, Disk[{0, .5}, .5]}, {b, Disk[{.25, .5}, .5]}, {ab, 

   Disk[{.5, .5}, .5]}}]