How do you tell if haircolor is expired?

Most permanent color is ammonia-based. If ammonia-based products become contaminated or exposed to air, they oxidize prematurely and won't process correctly when applied to the hair. Peroxide also will lose effectiveness when contaminated or exposed to air. Often times, if left sitting, the packaging will begin to break down and will compromise the quality of the chemicals inside. If you have permanent haircolor that has been sitting around for over a year, chances are, it won't work properly. The pigment molecules won't be damaged, so you still may be able to stain your hair darker with it, but it won't process correctly and you won't have the same staying power. If it is a deposit only semi- or demi- permanent haircolor, it should be fine.