Isoluminant

Having uniform subjective light intensity, where two or more colors appear to be the same luminance.

Colorblind individuals have a different Luminance Curve from color normals, so the relative brightness of different colors will differ. The Flicker Test is used to ascertain when two colors are isoluminant. It can be used to determine an individual’s Luminance Curve.

For isoluminant colors, the sensation of movement is greatly reduced, because information about movement is processed by the colorblind Magnocellular system; Depth perception is also hindered.

The top half is a gradient of green-yellow-red designed to be isoluminant to a color normal.
However, to a protan, the red appears much darker.
The bottom half is the same gradient, but the yellows and greens are darkened so that the gradient is now isoluminant to protans.
However, to a color normal, the red appears much brighter.