It's the M42 screw mount 50/1.4 that I'd like to use on my SP500. I got the lens off eBay and didn't realize just how yellow it was. I've heard about the "wrap in tinfoil" sunlight solution, but am concerned that would dry out the lubricants or somehow damage the lens. Supposedly there's an Ikea LED reading lamp that emits enough UV to clear the lens, but I'm doubting whether such a reading lamp really does emit that much UV.
No doubt I could use the lens for just B&W.
My Pentax SP500 was my first ever serious camera. It, and its 50/2 Super Takumar, was the only camera I owned and used for 15 years. So, I'm quite fond of it and still use it.