Yellowing is usually a sign of chemical contamination. Most likely, the monomer is reacting with traces of acid in the paper; if so, cloth towels should solve your problem. Another possibility is brand new brushes can be coated with a lacquer that yellows during initial brush preparation; in this case, get a fresh dappen dish of monomer after prepping the brush.
Both old and new brushes should be cleaned regularly according to manufacturer’s directions. Finally, don’t use the same brush for more than one acrylic system; cross-contamination may cause yellowing. -- Paul Bryson is director of research and development at OPI.