I’ve used the Chipex kits. An alternative is the Dr Colourchip kits which are pretty much the same as the Chipex kits. They are more expensive than the Seat touch up pen kits and they take a little practice to get good results (there are ‘
How to’ videos on the Chipex and Dr. Colourchip websites, and on YouTube), but the beauty is that if you’re not happy with the results, it’s easy to remove the paint you’ve applied with the supplied blending solution and then start again.
I used to find the paint in the Seat touch up pen kits can become thick after a few uses - when the cap is removed so the paint becomes exposed to air. When this happens, it can become difficult to apply without the application becoming ‘blobby’. I’ve not had this
problem with the paint that’s supplied with the Chipex kit though. As
@Rowan83 has said though, always apply with something like a cocktail stick (the end of an unfolded paper clip works too), rather than the in-cap brush in the paint pen if you’re looking to get good results, and build up the paint in the chip in layers rather than apply one thick coat of paint.