I got a tube of decorators caulk from B&Q, and a small selection of student quality acrylic paints - yellow, orange, Paynes grey, white. I also got a packet of empty plastic storage pots for 3 quid at Hobbycraft.
Mix a large dollop of caulk with some white acrylic (around 70/30) for the base, and then mix with other colours until you have a perfect match. It's really easy to apply with a small palette knife, excess wipes away easily with a damp cloth and it doesn't shrink. I've used to to tidy up some pretty horrific corners caused by a batch of warped/twisted moulding.
I find yellow/orange and a touch of grey makes a perfect light oak (a colour that can be tricky to find a filler to match), and it keeps pretty much indefinitely in sealed containers.
I've built up a few different colours for common frames and they can easily be tinted for one-offs.
Hope this helps someone!
