Melt 100g of Roberts Confectionery White Melting Buttons.
Add 1/4 tsp of Roberts Confectionery Oil Based Powder Food Colour for colouring chocolate and mix.
You can add another 1/4 tsp if you prefer a stronger colour. You are best not to exceed this ratio otherwise the chocolate will become chalky and change in texture.
If the powder dye looks like it hasn't fulled disolved or is streaky or grainy, it means the chocolate wasn't warm enough to begin with. Re-heat it gently and re-mix. The powder dyes need warmth to mix well.

If you are working in larger quantities, the chocolate make take on a chalky texture. To prevent this, you can mix the 1/4 tsp of Powder Food Colour with 1/2 tsp of Vegatable oil first, prior to adding to the chocolate.

Note, that if your White chocolate is actually quite cream to begin with (different brands have different shades of "white") this will also impact on the strength and depth of colour achieved. You can whiten chocolate first by mixing 1/4 tsp of Titanium Dioxide (White Powder Food Colour) into 100g Chocolate. This will give you a white base to start with. Then continue adding 1/4 tsp of coloured food colour.

If you work in a larger batch size, you will end up with a more diluted colour. This is due to the fact that you are applying a small amount of colour to a large volume of white.

NOTE: If colouring chocolate BLACK, you need to work in small batches, using the above ratios, to get a true BLACK colour result. If not, you will end up with a more purple or mauve tone of colour (as this the major undertone in the black powder dye).