This is an older article (PlayStation 2!) but the pics and text are great, worth a read. https://www.gamasutra.com/view/feature/2987/tool_postmortem_climax_brightons_.php?page=2
This is probably achieved with vertex paint of 2 materials. Check out how env artists are making those blendings in UE4. There are cool breakdowns on YT from CG society.