These examples show how you might re-classify a thematic raster images. You can also use raster classes to add to normal legends. A couple of things to note:

1) Each raster class must contain a name and a valid color to be used in a legend.
2) A class with no color defaults to the actual image color for that pixel value(s) in the image.
3) A pixel value that doesn't belong to any class is handled like an offsite pixel and is not drawn.

map file

Unmodified images.

Just the forest class drawn with normal color... Just the forest class drawn in yellow...

No forest, all others normally... Turn building and other annotation white...