This is something that's new to me. I don't usually worry about how much value contrast my colors have. In weaving, and especially in a pattern that's essentially an optical illustion, like Log Cabin, value can matter a lot.
Value isn't about the color's hue, it's about how dark the color is. The easiest way I know to check for value contrast (and I should have!) is to take a photo and change it into black and white.
As you can see, the values of my two colors were almost exactly the same. So, there wasn't enough contrast for the Log Cabin to really show up.