Sentence:  It is all the more remarkable therefore that in one respect — <WSD>weft</WSD> colours — some of the pirnless looms are more versatile than conventional machines. Figure 6 shows the colour mechanism of a conventional loom designed to weave six colours of <WSD>weft</WSD> (there is never any problem about colour patterning in the warp). 
Meaning of the word marked with <WSD> tags:  The horizontal threads that are interlaced through the warp in a woven fabric.