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I'm using a green LED now, I wonder if a blue one would work?

It's not like a higher frequency wouldn't work, it's just that it probably wouldn't be operating at its maximum brightness. I've replaced red LEDs with white ones and they look fine. It's possible that if you replaced a high-voltage (high-frequency) LED with a lower-voltage one you'd run the risk of exceeding its rated current, but in general it's very hard to blow up LEDs.

Peter