I’m a big fan of soap pigments. They tend to colour my soap very evenly and are easy to use. They are definitely a lot less messy than lakes and micas, and also a little goes a long way. I would strongly suggest to invest in a trio of soap pigments, if you are into cold process soap making!
I use soap pigments from Pure Nature, which are available for $12 per 10 g. They are a bit more expensive than the lakes and micas, but are a lot more economical and will last you for a long time. They stock the basic three colours: yellow, red and blue, which can be easily mixed to create more colours, such as green and blue.
Soap pigments are water-soluble and very easy to work with. The pigments are mixed with water before use, and then added to the soap at trace. If you pre-mix them, you can store them in the fridge for a week. I use these little mini-bottles from Systema for my prepared colours.
I use these bottles in my workshops, and it’s basically add and mix until you get the colour you desire.
The recommended usage rate for these pigments is 0.02%, which is really a very tiny amount. But I found that you can easily double the usage rate to 0.04% without any trouble – meaning no staining and no coloured foam. To calculate the usage rate into workable amounts, I added 2 mini-scoops (0.15CC each) to 3 teaspoons of water, which gives me exactly 0.02% (MATHS: 0.3 ml / 15 ml = 0.02). Add 1 teaspoon of the solution to 500g soap for a 0.02% usage rate, and 2 teaspoons of the solution to 500g soap for a 0.04% usage rate.
Or in other (non-mathematical) terms, you need one 0.15CC scoop to colour 500g of soap!
As mentioned earlier, the pigments can be easily mixed to create more colours. For a green, mix one part blue with one part yellow. Add more yellow for a more yellowish grassy green, or more blue for a teal colour. One part red and one part yellow will make orange.
Purples and lavender are a bit more difficult to get right. Purples tend to be more toward the red end of the colour spectrum, so mixing 1/2 red and 1/2 blue usually doesn’t work. With the pigments, I found the ratio 2/3 red to 1/3 blue worked well for me. If you wanted a more reddish purple, you could even increase to 3/4 red to 1/4 blue and increase the usage rate to 0.04%.
Adding titanium dioxide will whiten your soap to make your colour appear more true, as you can see in the picture below. To get the shade of lavender, I added 1/2 teaspoon of titanium dioxide to 500 g of soap.