First, make sure you have upgraded your version of ReMask to 3.2.1 or higher. See here on how to upgrade your plugin and check which version you are running.
In ReMask you might notice that the 'Foreground Color' (high quality compute, recovery, desaturation) panel is grayed out. To have access to this panel, please make sure you have computed your mask and then make sure one of your view modes is in 'Keep'.
You might also need to check under Menu -> Preferences that 'Enable High Quality Compute' is selected.