I tried Cedrics approach to color the scene fog. Used a small reflection probe, actually 4 pixels should be fine, too. Sometimes the fog lights up though.
var rp = new ReflectionProbe("skyReflection", 16, scene)
pixels = rp.cubeTexture.readPixels(0,0)
// i take the first pixel of the reflection probe texture for fog color.
// since pixels are stored as buffer array, first pixel are first 4 values of array [r,g,b,a....]
scene.fogColor = new Color3(pixels/255, pixels/255, pixels/255)
hope it helps some other people.
Edit: Maybe it is necessary to assign the cubeTexture to some object material in the scene. My reflection probe texture was only updating, when this object was visible. Assigning the cubeTexture to scene.environmentTexture does not seem to be enough.