Visible Letters was developed to help a child with a cerebral/cortical visual impairment (CVI) learn to read. He struggled to distinguish letters by shape, so this app allows letters to be different colors as well. For example, he couldn't tell a 'K' from an 'X', but when 'K's were yellow and 'X's were red, he could tell them apart.
You can type text into the app directly, or open a plain text file from Google Drive (Visible Letters should be one of the options presented to open the file.)
Sizes and colors can be controlled from the "Settings" item in the option menu. You can set the default color and size for all letters (and numbers and symbols), which will be used if you don't specifically set them for a letter.
There is also an option to "Move Letters" from the option menu.