If you don't seal the leather, oil dyes can transfer to porous handle material. If you choose to 'store' your knife inside your sheath (leather or kydex), the you need to maintenance your knife (WD40 draws moisture out, mineral oils rehydrate your porous handles/carbon steels). Leather will draw moisture out if your porous handles .. which is probably why you're seeing a lightning of color from where it's in constant contact with the leather. You're doing right by using mineral oil to rehydrate.
Personally, I thought CPKs were user knives. Designed, built, bought for just that. If you're buying them as 'collector status' .. then oil them, wrap them in wax duck cloth, and put them in a UV protective cabinet. - and dont ever put them in a sheath (leather or kydex).