Congo red comes in two variants one is >37% and second with >97% dye content, now question comes whether the dye content available affect the kinetics of binding hence giving false negative results even if amyloid is present in sample?
It will not affect the kinetics, but staining intensity may well be affected if your dye concentration is only 1/3 of what it should be. Another question is what effect the impurities have on the staining. And finally, you open your study to criticism at the publication stage if you use impure reagents.