Hey, if anyone was curious about this, I finally found some paint and a little brush and the time: I'd been putting it off till my hands were a bit better for fine work.
Some acrylic miniature paint made for little war-game miniatures that I got for a little project years ago was still alive in my crafts box, yay. It has the unlikely name of 'Skull White' by Citadel Colour and is really almost impossible to distinguish from the other white on the body. (at least without direct popup flash for lighting: I couldn't get the reflections from it off the surrounding parts of the switch.
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There's four grooves in there, but once I painted one, I could see that was sufficient, so I just did the two middle ones cause I thought it'd look nice. Came out looking pretty sharp, and it does what I'd hoped: the switch no longer disappears from sight when I'm not looking right at it.
No more forgetting I've left spot or matrix metering on, hopefully. (I'm just not used to looking at the back of the camera up there, I guess, there's usually nothing to see from that angle but a rewind lever.
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Pretty easy to lay a little paint in those grooves and wipe off anything not in there. We'll see how this stuff stays, which of course was the original question.