Red Variables

I am helping out on a project from the Lowell Observatory on young M class dwarf stars, trying to determine the rotational period of a number of selected objects. The assumption is these stars have large spots, which makes detection of a rotation period possible.  Being M class objects, they are of course very red, as is several (of the very few) available comp stars in the various fields.

I know there is a general prohibition about using reddish comp stars, but how about for for red stars themselves?    I am thinking since I don't need a precise corrected magnitude, only a differential magnitude to measure the light changes, it shouldn't make all that much difference.

Any thoughts?

Jerry