I seems to be a bug, thanks for the report. Some colors are sometimes incorrectly "rounded" down. As a workaround, you can switch from "Smoothing" to "No smoothing" mode in the toolbar above the canvas while using this tool. In that case, it should preserve the color.
Select => color losses (RWPaint)
Make a new image, color it with 1 color (not black),
click on "select" tool, and pick the first one. (to rotate the selection)
Select a zone, unselect it and check (color picker) if the color is still the same. (it has changed isn't it?)
Is it a bug?
How can I prevent RWPaint to do that? 😉
Or, how can I rotate something 90° without using this tool?