Have you checked the status of your ink cartridges - even if not 'empty' one
may have gone bad? That's the first thing I would suspect.

If that isn't the problem you might determine whether there is a driver
update available &/or reinstall your printer driver, as it is quite unlikely
that Word is causing your problem. There is also the possibility that the
printer, itself, is malfunctioning. How does it do if you print from other
programs?

If everything else checks out, you'll need to supply more specific info:
What version of Word/Windows? What do you mean by "mix" - some characters
red/some black, or individual characters printing as a mixture of red &
black?