How to paint seat tube panels

I'd like to do some frame painting, how do I transition from 1 colour to another without leaving a ridge where the masking take was?

Been googling for guides but most stuff is for painting house walls

The marsking take is usually used on the second color. So You would paint the hole bilke pink (say), then maks the bit you want to be pink. Then spray the rest.

just as cip is saying will do the trick. Masking tape has a tendancy to bleed the paint through, which means that nice straight line you have masked can and probably will be ruined. Best bet it to get some fine line tape 3M must do some. A nice thin coat on the 2nd colour and a nice thick clear laquer coat will leave you with no step from colour to colour.
Currently in the process of restoring a frame myself.

any metal paint you can get from halfords or b and q. Hammerite will really do the trick. Just make sure that you use a good quality primer (not cheap shite)

But, do you want to have defined lines between one colour and the other? Or do you want sick fadez?

The tape I think you want for the above methods is either the 3M precision stuff ("high performance") or Frogtape, which is apparently amazing (but I've not used).