I'm a henna lover and user for about a year now. I have used Jamilla, and henna for african hair from mehandi.com. I use only water to mix, but a lot of ladies here have had very good dye release by adding lemon juice, or another acidic base. The color that will be released will not be dark enough to cover gray strands. I've never used Karishma; but do plan on trying it in the future. My experience with henna and gray hair is that each time I henna, my gray hairs get highligted an orangey-brown color,
while the rest of my hair is dyed a beautiful auburn with brownish red highlights.
If your mom's goal is to cover her grays, then I strongly suggest the 2 step process: henna first, then indigo. There are many excellent henna threads around here to help you. Also check out the link-
http://www.hennaforhair.com This site is awesome!!!! Good luck to you on doing your mom's hair! I know she's going to love it!!!