I've implemented this by setting the button's background colour to the highlight colour of the digit, and then when you select another number pad button, reset the background to DarkRed.
For the next versions of both SuDoku Solver and SuDoku Lite, I want to move more towards using the full features of the Holo Dark theme which Android pushes as a key part of the "Android experience". Using attractive shades of darker or lighter gray, with a black background, this is designed expressly for mobile devices to help save on their use of battery power. I've used many features of Holo Dark in the released versions, and the major hangouts so far are the use of colours on the grid, which I intend to keep, and the dark red shade and lack of rounded corners in the screen buttons. So I've spent the last day or so working out how to exploit Holo Dark buttons while still being able to set the button background colour to that of the selected candidate digit when one is selected (which is easy) and then set it back to the default when a different digit is chosen. This latter has proven surprisingly difficult, and in the end the only way I found to solve it was to import the standard Android button backgrounds for the various states of a button (normal, pressed, focused, disabled) and set up an XML file that specifically displays the correct background image as the state changes. Then, when a number button is deselected, it is necessary to reload that XML file as the button's backround resource. [I can assure you this all makes sense to an Android developer!]
That is now working in both products. A further consequence of the change to using the full Holo Dark feature set, however, is that the buttons are slightly smaller than they were before, as Holo Dark insists on a greater gap betwen buttons than I had been using. Quite reasonably, too.
I now have two users of the Lite version, and no error reports yet. Both are using different tablet device types, so any feedback will be truly useful. There's only so much you can do with the Android emulator.
At that point, I need to head off to Holyhead tomorrow mid-day to get ready for the launch of my boat on Wednesday. So far the forecast is looking reasonable. The wind will be trying to push me onto the cliff face just to port at I come out of the crane backwards, but probably not strongly enough to cause any difficulties. And I've already run the engine since changing all the filters, so there shouldn't be any problems there. (It's a bit embarrassing when the engine doesn't start after a launch. Been there, done that, didn't bother getting the T-shirt.) The last remaining problem will be picking up my mooring single-handed. Oh, and keeping the dog out of mischief.