When it updates the red channel, the Simple Construction Script (SCS) gets re-run (via AActor::RerunConstructionScripts) on the Blueprint Actor instance generating new instances of all its components. This leads to UEditorEngine::NotifyToolsOfObjectReplacement getting called as there's a mapping of the old components to the new ones. Which ultimately leads to SDetailsView::PostSetObject getting called, which has specific code for closing the color picker if SColorPicker::OptionalOwningDetailsView is invalid or doesn't match the invoking SDetailsView. SColorPicker::OptionalOwningDetailsView isn't set when the color picker is first opened in FCustomPrimitiveDataCustomization::OnMouseButtonDownColorBlock, so this means it's always closed.
issues working with the color picker and StaticMesh custom primitive data on Blueprint Actor instances in editor.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-193655 in the post.