HR diagram x-axis (color) and redshift?
The short answer is that the redshift of stars that are close enough to put into H-R diagrams (or &quot;color magnitude diagrams&quot;, if we're being precise, given that you're talking about using colors) is so small that the effects are minimal.
The correction to color you're thinking of (called &quot;K correction&quot;) depends on the redshift: the smaller the redshift, the smaller the effect.
But in order to distinguish and measure individual stars (so as to construct an H-R diagram), the galaxy has to be close enough -- the upper limit (for observations with the Hubble Space Telescope) is about 10 Mpc, or a redshift of $V \sim 1000$ km/s. For that kind of redshift, the K corrections will be $\sim 0.01$ magnitudes or smaller (you can test out different values using this online calculator -- note that it uses $z = V/c$ for redshift). This means the effect will be smaller than typical observational errors.
Put simply, galaxies that are far enough away to have large enough redshifts to worry about the effects on colors are too far away for you to observed individual stars and make H-R diagrams.
For stars within our own galaxy, the redshift (or blueshift!) effects will be due to relative velocities $&lt; 200$ km/s or so, which means the effects will be really small.