using water-based touch-up paint

I've got a 24 year old car, in dark metallic blue. I'm pretty certain there's a lacquer over it as well. It has a variety of small stone chips, rust bubbles etc. I also have a pot of touch-up paint which is water-based. When I initially put the paint on, it's an excellent match, but as it dries, it seems to go much darker. I also have a little pot of lacquer (like a nail varnish pot) and I'm not sure if it's water-based or not.

Could anyone give me a few pointers please?

Why does the touch-up paint darken a day or so after it's been put on? Should I put this lacquer over it? If so, how long should I leave it?

Many thanks!

a 24y/o car WILL have faded with age, a ready made tester pot will be the original colour, not the faded one.
Id go to a body shop and get them to make up a matching touch up pot, will cost more, but it will match the faded colour

Thanks Tim, Will try that.