For instance, if I have a TextBlock:

"Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum."

I want the first 10% of this TextBlock, such that the font color should be red, and the rest should be green.

This perhaps means it would color the "Lorem ipsum dolor sit amet, consectetur adipisici", and part of the "n". Basically pixel-wise font coloring instead of character-wise.

Another important behavior is that the percentage respects the wrapping, in that if 50% of the TextBlock contents is colored, it should mean 50% of the text in reading order is colored, and not the first half of the block.

For example, this is a similar question (with correct answers), but regarding Label/TextBlocks with no wrapping: http://stackoverflow.com/questions/2394421/is-it-possible-to-seletively-color-a-label-in-silverlight