When coloring over grayscale the gray serves as your guide. Think of it as an adult color by numbers without the numbers. Instead the darkness or lightness of the gray tells you how dark or light a color to use and where to apply it. Simply put, light colors over light grays, dark colors over dark grays and medium colors in between to seamlessly blend light and dark.
Some points to keep in mind:
Here is a little cheat sheet I provide at the beginning of the "Beautiful Creatures" grayscale coloring book:
I'm a Mom of 2 from Vancouver, Canada. I recently discovered grayscale coloring and loved it so much I published Beautiful Creatures and Beautiful Nature because there were no high quality grayscale coloring books to be found. Join me as I write about the grayscale coloring techniques I have learned and that I am continuing to discover!