Tux Paint is not meant to be a complicated paint program, so enhancing the Text feature much (beyond the introduction of the new Label variation of the tool, coming in 0.9.22) is not a high priority. That said, it's possible to create a line of text using a variety of colors, fonts, sizes, etc. by using the [Tab] key to end the current text and begin a new one immediately next to it. (Versus using the [Enter] or [Return] key, to begin a new one on the next line, at the left (or right, if RtoL language is being used)).

Thanks, though!