There's more than one cause for that.

It could indicate that the PH is out of range.
Could be something in the soil. (see above)
Could be Phosphorus lockout. (see above)
Could be sunburn. (see below)

Depending on the strain, it could be inherited.

If they look otherwise healthy, I'd advise watchful waiting over "fixing".

Aloha,
Weezard
Weezard Reviewed by Weezard on . first grow log So this is my first grow log . I have here five big bang 2 seeds from gorilla seed bank an one white widow an one purp plant all from gorilla seed bank I germinate for two days in a glass of water . When I seen a small piece of root on all them which was about the end of day two I put them in a paper towel till the root was a inch or so . Then I planted them in some small pots with miracle grow seed starter soil . I use bottle water cause my tap water is to high in ph its over 7.5 so with Rating: 5