answer: they don’t, but you in your small mind are overgeneralizing.
My mikvah has blue soap. My other mikvah has no soap.
The color is not important, the chemical makeup is.
My kids like blue slurpee, not raspberry. My kids like red slurpee, not cherry slurpee.
You must be a 4 year old, who should not be going to the mikva.