On a Mac, Following the tutorial located here: https://answers.unrealengine.com/questions/132223/macbook-pro-preview-rendering-issues.html
When opening a new level, the atmospheric fog renders solid red. Adjusting the Density Height in the Precompute Params alleviates the problem (unless you delete the atmospheric fog and start over.)
On a Mac, Open a blank project in UE4.5.1
Open a new level
Add BSP ground plane to grid
Add Directional Light
Add Atmospheric Fog
Note the Fog renders almost solid Red
Adjust Density Height in the Precompute Params
Fog will now render correctly
Head over to the existing Questions & Answers thread and let us know what's up.