Choose a color scheme.
Break colors into a value range from the lightest to the darkest.
Complete the worksheet:

Your photo is made up of 4-6 different values.
Analyze your photo and count all values. This step is important!! You can number them directly on the reference image.
Then select a color scheme.
Mark your color scheme on the worksheet.
Premix all colors before you start painting.
They should be of obvious different values.
The lightest value should be the lightest possible tint - add your color to White, not the other way around.
Also have some amount of black and white paint - based on your image.
Try your colors on the worksheet.
If you need to adjust a color or two - time to do it now, before you paint your project.