The iron content in the water up here in Michigan is pretty high and I can see where it has effected my dyeing. While in Texas, I dyed using dandelion heads and got some very bright yellows. I used the dandelions from here with the same mordants as I used in Texas, only to have the colors come out a bit more subdued. It could be from the water or perhaps the soil differences. ORG Finally found my snippet of info on mordants for dyeing.
Color is influenced by the choice of mordant.
Often combined with cream of tartar gives bright clear colors. Chrome and Tin - Chrome gives color depth and greater permanence, and tin brightens tones. Again both are poisonous. I think you are supposed to use glass or enamelware basins for your dye baths, because an aluminum basin would add its' chemical components to your dye bath even if you didn't want it. So your stuff would not come out quite the way you might want it to.