Knorke. They aren't lava patches. Look at the map in fps mode and take a gander at the sky.
That knowledge, plus the knowledge that the metal patches are actually features cunningly shaped to the exact size of my metal patches should explain a lot.
If that still doesn't do it... Turn on reflective units, load up a mod of your choice, and zoom in on any units. Then ask yourself why the texture has a reddish tint

We're getting warmer
Nah I'm just messing with you dude. The reason theya re red is because the sun has a reddish tint and the features reflect the sun. Features make the best looking metal patches, but they aren't all that customizable which is the downside, but one I'm perfectly willing to take, because basically... I like them that way.
Thanks for the ncie comments guys ;P