there might be either (without picture is hard to say)
pigment has not been completely mixed into/with the putty, little "clumps" are squished during application resulting in streaks.
to avoid that - it is not enough just to mix pigment with lime putty or mix with water and add to lime putty - pigment has to be ground with muller into a fine paste.
another reason could be the sand - Kwikcrete Play Sand might look fine and attractive - it is a bad sand for plastering - you need "plaster sand" for that - it is yellow to gray/umber color and very coarse in the bag...
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fresco painting video tutorials: http://www.FrescoSchool.com
Fresco School Video Channel
Contemporary Fresco Gazette
commission fresco or mural http://iLAdesigns.com
Contemporary Fresco Painting Resouce Center