Mixing colors can give a mix of anything. There isn't really any true guessing unless you know what a cross will give. It's never an actual color but may resemble one somewhat. Sometimes you can get pretty mixes, and if you know what you're doing can try and replicate it. New colors can come about that way.
I know if I cross a partridge rooster with a BBS (or just black) hen I'll get black or blue offspring (depending on hen color) with the males having heavy silver leakage and females having heavy brown (gold) leakage. Cross the leaky girls back to a partridge rooster and you get chicks who are leaky black with the occasional partridge. Pick the best specimens for future breeding back to partridge. That's how I added blue partridge into some of my flock. Took several generations. But black based is easy to cross into partridge and get back to partridge color.
I used wheaten ameraucana for my blue egg silkie project pen because the wheaten doesn't cause much issue with partridge. 2 generations down and the pullets are pretty much back to color. It improved the black on the cockerels. Getting back to conformation is still several generations off at gen 3. They do look like bad quality silkies now which is good. I have 2 mature ones in with a good quality rooster.
Apparently one of the partridge hens I'd got early on had ancestors that came from some mixed breeding. White chicks do very rarely pop up on me as it's recessive white.