The button’s colour for highlight and activate are set by the CLUT and the selected colour scheme (of which there are 4). Here, you can define the background, pattern, emphasis 1 and emphasis 2 colours. Think of the B, P,E1 and E2 as simply 4 colours for pixels.
Now, things should be consistent if your final DVD emulates the CLUT and colour scheme exactly as the original one.
You can examine the pixels of the subpic bitmap (these actually make up the highlights) in DVDSubEdit and actually export the subpic BMPs to an editor, add your highlights for new buttons and reimport them.
As for the foreground, you have got that down pretty well. You need to add them in your favourite editing app. I use GIMP but Photoshop is also very good.
DVDRMP doesn’t allow for importing the still as a BMP - you need to make a little DVD as per the guides on the site and then use the Replace with still function. There are other programs which allow importing a BMP directly (eg VobBlanker). But in any event, you will need to do also your subpics (for all displays eg widescreen and letterbox).