Oh yes Glenn Doman books says to use Red Colour Words. But I read some other parents says to keep changing the font, style and colour just so the kid is not memorising a certain font but really memorising the words. Maybe you can start of with all red words then flash again with all different colours.
The idea behind varying font and colors is this:
1. Keeping it varied and more interesting
2. exposing your child to different font styles and colors. Particularly important is the letter "a", which is usually written differently than it is printed (that's why we have comic sans font). This exposure therefore let's your child figure out what is the essence of a word, and that it does not matter what font, style or color it comes in.
It's actually a similar reason to why we encourage attaching multiple pictures to a word. Eg., different dog pictures will make it more interesting, and your child doesn't end up associating the word 'dog' with a poodle (or whatever that particular dog breed is in that one picture).