I make soap almost everyday, the colorants used in soap making are water soluble which do not work for soft plastic baits. You need an oil soluble colorant.
I have successfully used a lip balm colorant that is extremely potent. They are FD&C pigments blended with caster oil. For comparison 1 drop of these colors = 10-15 drops of Del's color
The only problem is the limited range of colors.
MMS - Lip Balm Colors, Oil Soluble