Name: Steve
Message: I have a pair of 80/20 cotton poplin/nylon coveralls that are (ugh) industrial mechanic green. I wish to make them blue. Can I overdye for the desired effect, or must I remove the existing color first?

You can't dye from a darker color to a lighter one, because dye is transparent. Although you can over dye blue with yellow to make green, you cannot overdye green to make blue, unless the green is very pale and your desired blue very bright or dark. If the green is light enough, you can overdye it to make a dark navy blue.

Beware of chlorine (hypochlorite) bleach. Although bleaching sounds like an attractive idea because it might remove the existing color, it is highly damaging to nylon. There are other chemicals that you can use to remove dye that are more gentle to synthetic fibers such as nylon. Any of the "reducing" type of discharge chemical can be used; avoid any "oxidative" discharge chemical. See "What chemicals can be used to remove dye?". 

The easiest reductive discharge chemical to find in the US is Rit Color Remover, which can be purchased at almost any pharmacy or grocery store. Unlike all-purpose dye, Rit Color Remover is an excellent product. It works best if used in a large nonreactive coking pot, but it's a lot easier to use the washing machine with hot tap water, so try that first. You will probably need several boxes of Color Remover for one washing machine load. Follow the instructions printed inside the box closely.

Not all dyes can be removed, but there is absolutely no way to predict whether this will be a problem for your industrial mechanic green coveralls. It's certainly worth a try. The lighter you can get the dye color, the easier it will be to change to a different color. If you can't lighten the color, then black may be your only option for a color change.

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