Every standard volume pot will have the same effect on a pickup's brightness at an equivalent resistance value.
The range of a 500k pot is 500k max, down to 0k when rolled off.
The range of a 250k pot is 250k max, down to 0k when rolled off.
Both knobs, when rolled down to, say, 100k, should make any given pickup sound identical.
If you want to retain your brightness when you roll down your volume, look into a treble bleed circuit. They're cheap and designed to solve this exact problem.