Message: Hi. 

I have a pair of pants that is 70%polyeser, 30%viscose. I did not see the label that says 'Do not machine wash, tumble dry. Dry Clean" so I threw it into my washing machine. Now after the wash, it is no longer black - it is dark blue. I need it to be black because it comes with a matching jacket - the current state makes the colour mismatch very obvious. 

My question is will I be able to dye a pants of this material with your instructions?

Did washing damage the pants in any other way, other than by washing out the improperly-applied dye? Sometimes clothing marked "Dry Clean Only" will shrink out of shape when washed. If your pants look fine, aside from losing their color, then I suppose you can consider them washable now. Be careful not to wash them with anything lighter in color, since additional dye is likely to bleed out from the fabric. You cannot dye anything without a lot of washing, so there's no point in even considering dyeing something that is truly dry-clean-only.

Unfortunately, polyester is not practical to dye at home, and few if any commercial dyers are willing to consider custom-dyeing a polyester garment. Although cotton and viscose rayon are easy to dye in cool water, polyester requires boiling with a special kind of dye called disperse dye. Even if washing did not shrink your pants, boiling them seems likely to do so.

Your IP address (which was included when you sent your message) indicates that you are in Singapore. I'm afraid I don't know much about what types of dyes are available in Singapore. I can tell you where to mail-order disperse dyes for polyester from Australia, Europe, or North America, but I don't know where to recommend you obtain your dyes in Singapore. Do not try to dye polyester with any other sort of dyes, as disperse dyes are the only type that work. All-purpose dyes and batik dyes will not stay in the polyester.

Perhaps, if the jacket is unlined, the best move might be to wash the jacket, as well, hoping that it might fade in just the same way as your pants, so that the two pieces will match again. However, if the jacket is lined, this would be an unwise action to try. When a jacket is lined, the outer layer of the jacket will often shrink much more than the lining, resulting in an ill-fitting lining that ruins the shape of the garment. In addition, there is no guarantee that two separate pieces of clothing will react in the same way to either fading or dyeing. The two pieces may have matched perfectly when purchased, but may have come from different bolts of fabric which will react differently to washing or to dyeing.

I'm sorry that I cannot give you better news about how to fix your faded clothing.