Hi, Faust,
wlcome to
RP!!

... and thanks for the great Tip!!!
Rich,
It happens while painting with the mouse as well ... I don't know if the Wacom Tablet has a special command/option to prevent this ... I know the mouse doesn't ...
That said, it only happens if your 'brush' is set to an Opacity other than 100% because as long as you keep your pen down (or the mouse button pressed)
PS still regards it as your 'first hand' of colour even if you go over areas you have coloured already ... but, as soon as you lift the pen (or release the mouse button) then start again, is just like giving a 'second hand' of colour which increases the saturation when overlapping with the first...
As far as I know, the only ways to prevent this are:
* either not lift your pen until you are finished colouring the selected part (erasing the possible colour 'bleeding' afterwards),
* or, provided that you are painting on a different layer, following what
Faust said ... keep your brush to 100% Opacity and 'play' with the Layer's Opacity ...