Kite_Boy...........here are a couple of things to try:

Here in the USA.......at Walgreens Drug Store we can purchase a temporary hair color that shampoo's out whenever you want to remove. I use it with great results ! Simply apply to the ends of the unit, maybe a couple of inches up from the tips, and carry down into your natural hair. Hit it with hair spray to lock it in.
The instant product rinses in and shampoos out ! The name on the bottle is "fanci-full" I bet you can find something similar in your location.

Second suggestion is to use a spray on hair color............"TOPSOL CHROMOTONE......SOMETIMES, SEE IT ON EBAY. Otherwise do a search! I have used the chromo hair color spray as needed for over 30 yrs!! Also sold at another toupee supplier here in the states, but I hesitate to name on this forum, out of respect for BA forum.

Good Luck !!...........

johnny/abq

addresses on fanci-full bottle:

COLOMER U.S.A.,
JACKSONVILLE ,FLORIDA32254

also..........

COLOMER B.P.P.,
ARAGO 499
08013
BARCELONA, SPAIN