Hi Tom,
Try running r.support on your single band images before you use i.composite.
In the r.support command there is an option to update the histogram; answer yes to
that option.
Now try i.composite. It shouldn't give the WARNING message anymore and your composite
image will look much better.
Good luck 
Cristina Seabra
UNINOVA
New University of Lisbon
Lisbon, Portugal
cis@fct.unl.pt