well as far as I know, it's one of the cheapest way to make an artisan keycap. That amount Epoxy putty is not a big deal in budget and you only need a keycap which has a decent stem on it.
I painted them with acrylic paint with a small cheap brush (and put some clear-colored top coat on it for durability). again I'd certainly say that it is the best way to make a artisan keycap in terms of money efficiency. BUT the problem is the time and effort. unless you have enough time and patient (and some motivation craving for a unique keycap), it's probably one of worst ways to get a new keycap.
I personally think one of the great advantage of making keycaps with putty is that you need only 3 things ; hand, any keycaps, small amount epoxy putty .
In terms of tips, there is not much thing to say. the experience making keycap may be your great teacher.
I started with slime monster keycap which is very simple one to make. and painting it helps you a lot to improve your skill!
If you have enough money, I'd recommend you to buy an airbrush. it is helps you a lot for time and effort.
why do i not use airbrush? I'm living at an apartment, and my neighbor and family will complain me about the noise and smell if i use it.
If you have any questions , ask me anything you have about making keycap with putty.
