I've had a hard time learning about color values. The easiest way I have found to do this is to take a digital photo, download and then convert it to a black and white pictures. You can tell right away where the darks and lights are and if you need to move something.
Here is a sample of what I did for the scrappy log cabin. I just wasn't at all sure the values were going to be distinctive enough.
This has helped me. I hope you will find it helpful too.
quite obvious there are lights and darks