Here we have an excellent example of washed-out, frayed, patched, worn-down grey trousers, with the single redeeming feature of being very comfortable.
Who knew the dying process was so lengthy? Place dye and salt in the washing machine, place fabric on top. Wash at 40 degrees C, then wash again at 40 degrees C with detergent, then, run the washing machine again at 40 degrees C without the dyed fabric. This is to cleanse the machine of all dye remnants. Meanwhile, dry your newly coloured fabric.
It's like a totally different pair of trousers...
Interestingly, the dye doesn't work on the synthetic fibres, so the lilac stitching and scrap of pink ribbon bias binding inside are both intact. The colour also lives up to it's name 'black velvet', the black is so velvety you can almost feel it.