It is one string, colored once. You could create several instances of a text object, one for every line of text and set the text and the color seperately. You could pick the right text line by using a pv set to the line number of that text line.
Oh actually If there was an easy way to do it It would have helped me a lot on my project too. Beceause on my project I give player a text. Than I want player to write it fast. If he writes a letter wrong than change that letter's color to red if its true change color to green. It would be perfect.