I'm probably not the first person to ask this and I'll probably sound as dull as a box of used crayons, but what is the best way to colour white modelling chocolate?

By using powder colours before I mix in the corn syrup or afterwards when it has set with gel colours?

I'll be using the chocolate for a cake due on Saturday so any advice would be greatly appreciated.

TIA

Any method will work - you might choose by which colors you prefer. For example: some gels are more neon than powders, some powders are stronger than gels or liquid concentrates (especially with reds and purples, it can take a lot of gel if you want an intense color).

When coloring product I often use what I call the seed method: make a small amount of very strong color, then use a little of that to color the modeling chocolate as you use it. That way you can get the same color blend each time without having a lot of a particular color left over. Then the seed can be stored for another use without taking up a lot of space.