If you cut wet grass, your hands will go green if you touch it - cut it when its dry and that doesn't happen, unless you're using a hover mower which mulches it up, when touching it will also cause greening - the bottom and sides of my gardening trainers are stained permanently green...

Chlorophyll is the green pigment that dyes your hands/clothes etc. It is soluble with alcohol if memory of school projects is correct.

It maybe iron sulphate used for moss killing is green in powdered form and normall turns the water green mixed for watering on. Is it a grass green or turqouise? Also has it got a rusty iron smell..!?

Maybe its a form you can sprinkle on or its part of a granular lawn feed and weed etc.