Its an art. Why?
Beyond the delicate amounts of color used, different resins react differently when color is introduced.
A Standard white resin requires a great amount of color. The color needs to overpower the white of the resin. If you don't add enough color, the white will bleach the color into what I can only describe as a pastel color.
Clear urethane resins only require a pin drop of color because the color is fighting nothing.
When working with solid colors, you need to make all parts from one pour to achieve exactly the same color throughout the resin.
Clear resins are different. Because light can pass through clear resins, smaller parts made from the same pour will appear lighter than larger parts because more light is able to pass through them. So you will end up with various shades of the same color with only like size parts being the same in color. You will need to have several pours, adjusting the amount of color used in each pour to match the shades up or the larger parts will be darker compared to the smaller parts and look out of place.