how will I avoid frying the status LED when using 18V instead of 9V?

[Mark Hammer]

I don't wish to drag this out any more than necessary.  But I'll just point out that useful LED visibility is the overarching goal.  Some contexts can make even superbright LEDs harder to see, and other contexts almost demand turning the lights down.    A yellow LED against a polished aluminum chassis, without any bezel, would require serious illumination to be easily visible in room light, though much less on a darker stage.  A red LED, with a black bezel, in a black, navy blue, or dark green chassis, could be quite easily visible.

I'm in the final steps of wiring up an 8-device multi-FX strip.  A guy was nice enough to give me a heavy-gauge folded aluminum chassis that he had planned to use for something but never got around to.  I decided to make use of it as a multi-FX, and a way to make use of a bunch of populated boards I had laying around.  I decided that, since in theory all 8 effects could be on simultaneously (in theory; in practice it would likely sound awful), I needed to use different colour LEDs to be able to keep track of what was and wasn't on.  So I went with two repeating cycles of 4 colours: green, blue, yellow, red.  I used my little LED brightness tester, and used resistor values to achieve a similar degree of brightness (the red and yellow needed 18k, the blue 15k, and the green 1k5 - they were old ones).  I left the chassis buffed metal, primarily to be able to use some easily visible rub-on lettering on the top.    That meant using  medium-high brightness.

It's always a judgment call, but the goal is always having LEDs be as bright as you need them to: no brighter, and no dimmer.