I had a similar problem and was able to fix it but it might not apply to everyone. I created a new level that had an Exponential Height Fog and had checked the Volumetric Fog check box. The color picker wouldn’t open just as described in the earlier posts. When I unchecked the Volumetric Fog box I was able to use the color picker again even after I re-checked the box. I don’t know why that was a conflict or why unchecking the box worked but it did in my case.