That is most difficult operation. At first, SSMS can’t show more than 5000 separate objects at the same time. In order to show more we have to construct “MULTIPOLYGON” or “GEOMETRYCOLLECTION”. That only the way to fit more objects into SSMS screen. However it is still limited.
In order to combine triangles in a single object we divide them in buckets (Line 106).
In this example I just making number of buckets approximately equal to a number of objects within each bucket. Making lower number of buckets will increase processing speed, but produce less colors. All objects in one collection will have the same color.
Also, I wrapped the last query in extra CTE to have more flexibility on results formation.
This is a fun post showing some of the power and limitations of geometry types in SQL Server and their display in SSMS.