The most reliable way to make sure your color stays is to re-batch a plain batch of soap and add the colorant then. However I've had some good results with some ground spices and Kelp without doing that, so I guess it depends on what she's using. I've read that with natural colorants you have to always expect SOME variation. I think it adds to the interest and fun! =)
I've used cinnamon, ginger, and cloves. Cinnamon was def. my favorite. Even if you use your own recipe, re-batching is possible at least with some formulations. I've done it. Some recipes won't melt right, and some will. Anyway, hope that helped! =)
Something else I just thought of... If the color isn't staying in your soap, you might need to use more of it, or the particular colorant you are using might not be able to hold up to the lye and you should try something else.