Hey all. I know that I can replace the clipping diodes in my overdrive with LED's, but is there a difference and size and color of LED? If so, what?

not in size (same semiconductor, larger lens, usually) but they do differ between colours.
Basically, the more exotic the colour, the higher the threshold voltage. Also current requirements to fully light them are up, but that's not a concern for clippers.
Red LEDs have "lowest" threshold voltage, orange and green are next, through to blue, or even white (although that might be a tricky one: there's a lot of different ways to make white light - 3 leds in one package (RGB), a blue LED with phosphor-conversion to orange, etc)
LEDs clip at a higher point than regular diodes, so they'll distort later (need more signal to...). When they do, they do sound more harsh, so I'm guessing the threshold is a little more abrupt than with regular diodes.

With so many LED chemistries available, try as many different colors and makes as you can. Also try them with a 1N34 or 1N4148 in series with one of the LEDs to make the clipping asymmetrical.

You could easily spend days looking through the combinations.

I see different tone attributed to LEDs from harsh to fuzzy so I presume the circuit makes quite a difference.

Also try adding a 10K pot in series with both LEDs, or just one.

So many options.

Take care,
-Peter

Quote from: tonepoetHey all. I know that I can replace the clipping diodes in my overdrive with LED's, but is there a difference and size and color of LED? If so, what?

i use a red 3mm and a 5mm green

Just bear in mind that using LEDs with a higher clipping threshold will require more gain to be able to yield the same amount of distortion.