The roots won't grow much longer as they are short rooted plants. Mine are an inch/2.5 cm long at most.
The reddest of mine are those under the lights. The further away from the lights the greener the leaves and roots are. This doesn't really apply in my main tank as the water flow from the filter moves them round, but in the shrimp tank the flow is slow so the plants hardly move. There is a clip on light on that tank with the strongest light being in the middle of the tank and that's where the reddest plants are. The ones at the edges are green.
In my tanks, red root floater grows like made. In the week between water changes it can cover the entire surface so that I have to remove handfuls at each water change.