Why Linedef Type 606 (Colormap) doesn't apply to FOF?

I've been creating this map where I'm using a lot of Colormap. However I tried to do and FOF (which works correctly) and I wanted to add Colormap to its top too. But of course, a sector can only have one tag. So I made another sector which had Colormap, which I set with tag 3 and gave the sector with the FOF tag 3. Then the Linedef Type 100 (FOF, Solid, opaque) I had in that sector had the tag 4. So the FOF should have the Colormap as Colormap is affecting the sector that makes it a FOF. I don't know if someone might understand it, but

If you must have a colormap apply to BOTH the area below the FOF and above it, you must make a duplicate colormap control linedef and give it the same tag as the FOF control linedef. This will make the colormap apply to the area above the FOF as well, as this makes the colormap apply to the sector the FOF is being put in.