This is an image effect that attempts to give you an idea of what red/green colorblind users may experience when playing your game. It's implemented as an "image effect", and so requires Unity Pro.
Caveats: Currently it just takes the average of the red/green values at each pixel and sets both red and green to that average, turning red-ish and green-ish areas yellow, but it should prove enlightening for some games. Also, it does not impact GUIs.
Note: It appears that this most closely simulates Deuteranopia (a form of "Dichromacy"), AKA Daltonism, which affects approximately 1% of males and affects red/green hue-discrimination, but not any skewing of brightness. The most common form of color blindness is Deuteranomaly (a form of "Anomalous trichromacy"), which affects about 5% of males and 0.35% of females. Deuteranopia and Deuteranomaly are related in that they are caused by problems with M-cones, but Deuteranomaly is somewhat less severe as it represents a defect in the behavior of the M-cones, whereas Deuteranopia is the *absence* of M-cones. Thus, this filter should exaggerate the impact of red/green color-cue reliance in your design for the majority of individuals who have atypical color perception. That said, other classes of color blindness may be affected more severely than is reflected by this filter because those other forms (Protanopia / Protanomaly; L-cone deficiencies) can impact the perceived brightness of various hues in the red-green region of the color spectrum, rather than just hue differentiation. If my understanding is correct, that means that -- for example -- a heavy reliance on subtle or dark red colors to convey information may not be adequately highlighted as an issue by this filter.
Blue/yellow colorblindness, and full monochromacy are exceptionally rare (up to 0.03%, and 0.00001% respectively for the most common forms in the gender in which the form most frequently occurs) so I'm punting on those.
An example of a shot with more blue:
And the actual code: