Hi James,
Here's what I had to do when I was dowsing card color: I would intentionally
practice moving the dowsing tool (pendulum or L-rod) to randomly appearing
colors. First, I would notice which direction seemed most natural. Then, I'd
practice that 100 or so times before doing a dowsing test.
So, before dowsing the color of unseen top card, I'd run through 2 or 3 face-
up decks of cards and practice making the dowsing tool smoothly swing the
desired direction. It's mostly a physical response that you're training. If
you've ever played a musical instrument for jazz or classics, you have to put
in weeks or months of practice on the basics.
1st) shuffle the deck. Put it face down, then pick up your dowsing tool... get
it ready to dowse.
2nd) with your other hand flip the deck over, and make your dowsing tool
swing the proper direction.
3rd) with same hand that flipped the deck over, take the top card off the
deck. Now you see the color of the next top card. Same process. Make your
dowsing tool swing the proper direction to match the new color.
4th) again, take top card off, see the new color, and make your dowsing tool
swing the proper direction to match the new color.
5th) ... and repeat 4th step until you finish the deck.
6th) Then repeat for new shuffled deck for as long as your hand holding your
dowsing tool can move smoothly. You want to train a smooth predictable
response.
The reason I switched to deviceless is that some days when I've been
physically working hard, my hands would respond differently. So, I found
that I seemed more consistent with deviceless. Of course, there's those days
when I've been mentally working hard...

... and my deviceless will
respond differently.
I still need to practice almost daily for deviceless, and keep track of my
current natural reactions.