Try to pick more intuitive colours if possible. My first guess had both dark and bright red, and I wasn't sure which were closer/farther (had to reopen the site in incognito to get and read the instructions again). A more intuitive gradient (perhaps between just two colours, e.g. white and red) would remove the need for instructions.
You can tap the question mark in the top left to see the instructions again! But yeah, I can see how the color scheme might be confusing. As Kluggy suggested, I might add a gauge on the side of the board which should help prevent that mistake
