Which text do you find easiest to read?
Probably the one with the largest color distance between the text and the background color. That is the one to the right.
How can you make sure you use big enough color distance so that even the audience in the far back can read the text in your presentation?
One way is to use Mark Millers Highlight Explorer app where you can experiment with different colors for backgorund, foreground and highlighted text.
https://github.com/MillerMark/HighlightExplorer (it's just the source code so you need Visual Studio to run it though...)
If you got money to spend and want to know more about it you can take his course The Science of Great UI