My bet is that the color has nothing to do with neither magnesium citrate nor ascorbic acid, but either with impurities present in the the salt, or impurities present in water.
Note: these impurities can be safe, just giving unexpected color reactions.
Reddish/brownish sounds like iron(III), but it doesn't add with the presence of ascorbic acid. Ascorbic acid is a reducing agent, so I am not sure if iron wouldn't be reduced to Fe(II), which is not reddish. Checking redox potentials would give more info.