Hi Jure,
the same happened to my bass. It's completely normal. When you go the showcase and take pics of my bass (Bonnie SII 5 string) or other basses pictured there of some age, you'll see a more tanned overall look of the wood. The darker the wood of the bass the less it will be visible but it's still there. I think that even basses with ebony tops (see Rami's twins) will change colour during time. they kind of become mellower.
Stay at the low-end!