If you understand about basic colour mixing, i.e. blue & yellow make green then, I normally put an equal amount of dots of each colour on the stain and basically smudge it together with a lightly damp cloth, if not right then just add another colour to tint it, or wipe out with a wet cloth and start again !
Obviously its easier with a bit of practice, Always worked for me and doesnt cost the earth, just a packet of pens. Its not permanent until you heat set it, so you can experiment as much as you like until you get the right shade.
Steve