I finally made a detailed tutorial with the 4 steps to grayscale coloring!
I hope it will be useful and interesting :)

You can find it on my Gumroad

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As I know some people can't buy something on Gumroad via PayPal. Unfortunately I don’t know why it happens (someone told it was solved next day, someone just changed browser). If it you have this problem, you can note me and I'll give you my PayPal address and send the link to download without Gumroad x)