ImageMagick is a useful command line tool for manipulating images.
ImageMagick examples
Create a single-colour rectangle
convert -size 1920x1080 xc:red out.png
convert -size 1280x720 xc:7f0035 out2.png
# translucent -- rgb are in the range 0..255 but a is in the range 0.0..1.0
convert -size 700x300 xc:'rgba(255,0,0,0.2)' out3.png