I think it is probably the colorants.
I bought some Lake dyes which are supposed to be non-bleeding "unless you use too much." I didn't think I was using too much, but the orange centers of my white flowers bled until most of the white flowers were light orange.
I have had very good luck with Brambleberry's non-bleeding liquid colors and with their micas. Not a bit of bleeding, even when I use a lot of color.
So sorry about your soaps. They were so pretty! Don't let it discourage you... just try a different kind of color.