Probably one of the easiest things to do is Channel Filtering - "deleting" some of the channels so we can keep the image with the colors we want. For example, try this values (the bold values in red are the modified ones):
Channel
% Value
Additional Value
Red
100%
0
Green
0%
0
Blue
0%
0
Alpha
100%
0
The result will be an all-red image, since we filtered the Green and Blue values out of the image.