-Are there safe, natural dyes like henna or something I could use later on ?
The combo of henna/indigo can really color your hair dark/black/dark brown if you have the time and patience. I did a treatment a few weeks ago and my hair came out super dark and the little bit of gray I have was gone.

- Is it a good idea to color natural / kinky-curly hair?
I personally cannot use commercial dyes with the exception of temporary rinses due to the damage it causes on my hair. My hair is porous, medium to fine and the less the better.

- Will all the new growth I experience later on be white too!? D:
There is no way of knowing that until it happens. I started going gray when I was 16 (had one gray streak in the front of my hair) and by 20, the roots was dark brown again and the gray was gone eventually. My hair has been going through this gray on, gray off cycle for years now. Sometimes grays are dark again at the roots and it just grows out eventually. Everyone is different.
APL BSL MBL WL HL TBL | porous, fine/medium strands, medium density
My Fotki!
Below: Typical PS bun