Well, pots even when full on, still have a certain resistance. So 1meg pots will be as if your pots aren't there... and having 250k pots will have more resistance than the 1meg and 500k.. .so its a warmer tone.
Ppl use 250k pots mainly for single coils, since they are very bright. 500k for hums because they are usually warm sounding.
yes, if you use 250k tone pots for humbuckers, it will be warmer and less trebly.
I think if its a 250k for vol and 500k for tone (for humbucker pups) , you will have a slightly warmer tone (due to vol pot) but the tone pot will be like any other 500k pot for humbuckers.
I have a 500k vol and 250k fender no load tone pot. With the tone pot engaged, its a sligthly warm tone, warmer than my previous 500k tone pot. And the tone rolls off much faster and more. With the tone pot disengaged (bypassed), there's a distinct increase in brightness, obvious thru good monitors.