Heat needs something else to be relative to, in the simplest idea that a vacuum is empty then how can something be hot or cold if there is nothing else.
I think heat only really exists when there is something to be hotter or colder than. Are electromagnetic waves hot? I don't know, they have energy but is it heat energy until it is transferred to some other atom?
I love using coloured slip with a semi-matt white glaze over the top. I can't remember what percentage I used but 40% seems a lot to me.
Do some test going 5% 10% 15% and so on. Different colourants will require different amounts for the colours you are after. I paint my slip when the pot is still on the wheel. A heat gun works wonders for speeding up the drying so you don't smudge it while removing. Sometimes I paint on after turning but that is rare. I do use my coloured slip to stick on handles and it works fine.
I have never painted onto dry clay as I find it easier wet on wet. I have a video on my Youtube channel making the mug in my avatar if you want to see how I do it.
Don't put off the chemistry lessons for too long. Even if you want to avoid the chemistry there are a lot of glaze recipes that you can find online that would probably be fine for the job. Just look for ones that are for the temperature you fire too. It is a lot cheaper and you can get a nice white or transparent from 2-5 ingredients.