RGBColor
[r, g, b]RGBColor
[r, g, b, a]RGBColor[r,g,b,a]
is equivalent to {RGBColor[r,g,b],Opacity[a]}.
A swatch of color green:
RGBColor[0, 1, 0]
Let's show what goes on in the process of boxing the above to make this display:
RGBColor[0, 1, 0] // ToBoxes
A swatch of color green which is 1/8 opaque:
RGBColor[0, 1, 0, 0.125]
A series of small disks of the primary colors:
Graphics[MapIndexed[{RGBColor @@ #1, Disk[2*#2 ~Join~ {0}]} &, IdentityMatrix[3]], ImageSize->Small]