This issue has been closed as 'Won't Fix' due to an extended period of time without updates. If this issue is important to you please let us know by posting on the AnswerHub or UDN, and Epic will re-open the ticket for further review.
A licensee is having trouble getting the visual results they want because Lightmass doesn't seem to handle color changes from materials. They are seeing this in 2 cases: (See attached images and test project)
1. Vertex Color. When using Vertex Color and painting a mesh, the GI is not reflecting the new color, but reflects the default color instead (white)
(vc.jpg - Left platform has a red material which is reflected in GI. Right platform has a simple material with vertex color plugged into Base Color. Although it's painted red in the scene, the GI reflects the default white)
2. Similar problem with color based on World position. (vc2.jpg - Material uses world position and a HueShift to create random colors. The GI reflects only the default red and not the colors resulting from the hue shift)
Test project attached.
map.umap - shows the vertex color issue
map2.umap - shows the world position issue
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-20768 in the post.