Works better than I expected but lasts few shampoo only

I have short and thick dark brown hairs and about 10-20% grays focusing on the front. After the patch test, I used the product following the instruction. After 40-50 min, I rinse very carefully. In my case, this product covers about 50-60% of grays, and some of them look like very dark gray, not black. Fine for me, and it covers grays better than I expected actually. I feel that my hairs get dry a bit, but not seriously. The color seems to be pretty dark, almost black, but fine for me. I observe more grays (in almost white or very light gray color) after 3rd shampoo, so need to use my usual touch-up product a little to cover grays. After 5th shampoo, I can see more grays. The color remains longer than 5 days, but gradually disappears from 3rd shampoo. In my case it takes about 1.5-2 hours from the preparation and clean up, so I do not want to repeat this every week. I use this product twice (1 bottle) and I guess I the amount is enough for me to use it 3 and half times coloring. Here's my opinion. I may use this product if I need to cover my grays longer than 2 weeks but less than 4 weeks between salon appointments. I think it would be better for me to use my touch-up product for shorter than 2 weeks period and use usual color product for longer than 4-5 weeks period. However, if you have long and lots of grays to cover, this product still can be more useful than touch-up products. Smells fine for me (very minor) and easy to apply.

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