It is quite time consuming, but I like the result. If you have any questions, I am happy to answer them in the comments section. Also, please leave a comment with experiences making your own dice.
Here some details/tipps:
Mix two parts of enamal paint with one part of compatable paint thinner. This is the ideal compromise between fluidity and opacity.
Use a 20 degree v-shaped cutter with a point of 0.2mm and put some denatured alcohol for cooling on the die before milling.
A milling depth of 0,5mm is ideal.
Do not mind, if some paint swaps over while filling - you can clean that later.
You have to wait several hours for the paint to dry. You can shorten that time drastically by heating your kitchen oven to 50°C and put your dice there for drying.
Use paint remover or nail varnish remover and cotton swabs to remove spilled colour after the paint is completely dry.