Just so you know this kind of is possible as “Clear Outside” will set outside to whatever colour is currently set as “Background”. This is usually black. You can go to Edit > Options > Colors to set it to another colour.
If you want to set something outside of the name options in that menu then you can use the colour dropper to set RGB values for foreground and background (by clicking on the F and B boxes respectively). Setting a value like this would be a little odd though. Say you wanted the value to be 75, you would set each R,G, and B to 180 (255-75).
But to be honest, the way I would do it in practise would not be either of those options, I would make an inverse of the ROI using Edit > Selection > Make Inverse and then use Process > Math > Set to set the value of all the pixels outside the ROI to a specific value.