We should not use color for hair. We should use good hair oil containing aloe Vera or genoderma extracts. Healthy hair is better than sick and artificially blackened hair.
Applying mehndi is good,before the hair becomes white.If you see in starting,that your hair is going to become white in few days,then start applying mehndi on it.
Now-a-days i think kali mehandi is coming which is also herbal,try that or even henna works fine. I never used hennai as it turns my hair into red color if applied as already my hair is darkbrown in color.
Yes,this could again create problem,when applying mehndi on hair,as our hair becomes red. So,its better that you take care in early part of our age that our hair is not damaged. Need to take proper care in beginning,to avoid our hair to turn white.