This expression should work to get all the reds without having to do colourspace...

userbinator on Sept 10, 2016 | parent | context | favorite | on: Detect red circles in an image using OpenCV

This expression should work to get all the reds without having to do colourspace transform (standard unsigned 8-bit RGB expanded to signed ints of greater width):

    r - g > 128 && r - b > 128

The constants can be adjusted to change the definition of what you consider red. (Based on https://en.wikipedia.org/wiki/File:HSV-RGB-comparison.svg )

lettergram on Sept 10, 2016 [–]

It doesn't properly handle the light, i.e. if the r or g channel is super dim it'll cause issues.