Front tooth gray after root canal. Bonding or veneer?

At this time, even with braces on, your dentist could try "internal bleaching" which should whiten the tooth up somewhat.  After the braces off he can try a combination of external and internal bleaching.  Based on how dark the tooth is, bleaching probably will not lighten the tooth to match her other teeth, but should lessen the darkness. At that point (after braces) plastic composite bonding could be used.  Regarding porcelain veneering, there is no contraindication whatsoever to porcelain veneering a young persons tooth that has had a root canal.  However, most likely it will have to be redone when her mouth matures and her gums change with age. 

You didn't say if your daughter had a root canal done on the tooth or not.  If not, she most likely will in the future.  If she has, then you can do an internal or non-vital bleach that many times can return a tooth to it's normal color.  This is done by placing a very strong bleaching material into the chamber of the tooth that has had root canal therapy.  It can't be used with a normal live tooth.