I want to try making a horse white. How do you do a colour change from a bay, black, chestnut, to a white? How do you keep the details? How do you make it so the horse doesn't look brighter than the background? Any help appreciated!

Most people would tell you it's easiest to do that with a chestnut. Personally, I decrease the color saturation and alter brightness/contrast as needed. YMMV

This goes to a PDF about how I go about all my colour changes Sage. You can use this for black to greys or extreme white markings.

Thank you both!