On the "Color" screen under "Properties", there are four items to be selected with radio buttons. Ignore these for the time being. Select the color you want to change by clicking on it. The colors are in the sequence 0, 1, 2, .. D, E, F, but they are not identified, so you have to count to get the right one. Change it by clicking the up/down arrows or typing in the value you want. The values are for each of Red, Green, and Blue. They range from 0 through 255. That's where you get 16,777,216 possible colors.
Next, very important: Click on the proper color for the field type selected by the radio button. For example, if "screen background" is selected (the default), and you want the normal screen background to be green, click on the 3rd coolor (2). Otherwise it will be mapped to the last color you changed.
Then, finally, click OK to save your changes.
One more thing: You have the option of making changes to the current screen only or for the shortcut. The default is for the current screen only. So once you have closed the screen, you lose all the changes anyway. So remember to select the other option before you click OK.
Note that the COLOR command will supercede the radio button selections, but only for the "normal screen". So if you use the radio button to select color 2 for screen background and then run the command "COLOR 1F", the screen background will be remapped to color 1. Of course, color 1 could be 0,127,0 (green) if you want it that way. But I would do that only for an application that uses some ungodly color combinations (which I have one).