Does this work better to cover grey when hair is completely dry or still damp after towel drying?

I would be happy to assist you with the answer to this. Our Clairol Professional Beautiful Collection Semi-Permanent Hair Color should only be applied to damp hair. It will not coat the hair follicle if the hair is dry so you will not see any coverage unless the hair is damp. I hope this answered your question today. Have a Beautiful Hair Day!

If i have left over semi permanent color. Can I use it again?

This is a really great question that I would be happy to answer for you. Yes, these bottles are not usually used completely the first application leaving you product for other applications. Make sure you close the top and store in a dry cool place it will be ready to use on your next application. I hope this was of help to you and that I answered your question. Have a Great hair day!

Can I get a list of the Advanced gray coloring from lightest to darkest?

Black is 1 and white is 10, so with any haircolor those are the rules, so the lower the number the darker the color.

Will this be wash off totally? Wondering if I can return to my gray hair after some weeks

This is a great question I am happy to answer for you today. Yes, Our Clairol Professional Beautiful Collection, Semi-Permanent Hair Color will wash out of your hair naturally and will last up to 12 washes. Pro Tip: To get the last of the Semi-Permanent hair color to lift out of the hair you can use a Clarifying Shampoo. I hope this helped you answer your question today. Have a Beautiful Clairol Hair Day!