I know that some natural colorants, eg.; paprika, red sandalwood, spirolina, can leave specks in the finished soap. The trick I read about at the soapmakingforum is to blend the colorant into an oil like olive oil and you can save the colored oil to add to your soap when you need it. When you use it you can strain it through a filter like a coffee filter or cheesecloth. Another option is to put the colorant in a sealable tea bag, then drop the tea bag in the oil and let is set until you reach the desired color. The use the colored oil for soaping. Personally I found these methods too labor intensive for my taste but other soapers swear by them and use them for many of their colorants. Its a great method especially for annetto seeds. I actually embrace the 'speckled' look of some natural colorants and use them as part of my swirled colors to lend a look and feel of texture to the soap.