Relationship between metallicity and color? Should Pop. I stars be blue?
I think the misunderstanding (which I've been asked about before) is because Population II stars are redder as a population, even though for a given mass, a main-sequence Pop II (i.e. metal-poor) star would be bluer.  That is, if you compare representative samples of Pop I and Pop II stars, the Pop II stars will on average be redder.

This because Pop II stars are generally old so the blue stars have already died as supernovae or evolved into compact remnants. So looking at the whole population, you'll see more of the longer-lived, lower-mass stars (including red giants), which are redder.

The same argument applies to galaxies that aren't forming stars appearing redder than those galaxies that are still forming stars.  Since new stars are being born, some are blue, but they live fast and die young, so unless the galaxy keeps forming stars, it will become red overall.  Again, this isn't because of the stars being redder for a given property but because the population lacks young, hot, blue stars.