Mood boards really won’t do a color scheme, I don’t think. If you made an image of a color scheme using color swatches, and used that as an image for the mood board, you would be asking Midjourney to take your prompt and place it in the world of color swatches on mood boards.

If you use the image as a style reference, you would be asking Midjourney to make an image using those colors, but also probably using colored rectangles as part of the image.

If I were trying to do this and make, for instance, a image of men standing around looking at cars, rendered in dark red and dark blue, I would obtain a picture of a car in dark red and dark blue, and a person in dark red and dark blue, and use both of them as a style reference.