The thing to remember is every color has an inherent value. When a color photo is desaturated or converted to gray-scale, you are left with only the value of each color. Working in greyscale/values allows you to focus on creating convincing form and depth in your painting without worrying about color matching/mixing, etc. If you're working in Procreate a quick way to check your values is to create a white layer on top of your composition, and set the blend mode to "Saturation." When you toggle this layer on and off it will automatically convert your image to greyscale or the inherent values, so no matter what palette you're using you can see if your values are working properly. You can be a bit more free with your color choices as you explore different palettes as long as your values are correct! Here's an article from Ron Lemen on the subject. Hope this helps! https://www.muddycolors.com/2021/09/how-to-color-value-color/ And here's how to set up a Saturation Layer in Procreate to check your values https://youtu.be/8SxbDnGtc84