Implement direct sector colormaps with the sector fields lightcolor, lightalpha, fadecolor, fadealpha, fadestart, fadeend, colormapfog and colormapfadesprites.
In UDMF, 606 copies the colormap from a reference sector to tagged sectors. Useful for global/regional colormaps.
447 and 455 supply the colormap via a reference sector as well, because there's just not enough space to supply the colormap in the linedef args.
Using a reference sector has one major downside: If your map has a global colormap, it will override the colormaps in the reference sectors of other colormap linedefs as well. This means you can no longer change a global colormap. To fix this, I added a colormapprotected sector flag. If enabled, the sector's colormap cannot be overridden by linedef effects.