In modern art and media the Knights Hospitaller are nearly always shown as wearing near completely black tabards or surcoats. I remembered a lecture from a few years ago though, where a domestic historian/enthusiast who specialised in the medieval period claimed that black dye was the hardest and most expensive of all to make, and clothes in that colour were only worn by the rich. From some preliminary research on google, I've seen that some enthusiast sites don't really come to a strong conclusion about it.
So my question is, how inaccurate is our typical depiction of Hospitaller troops (including Knights, Sergeants, lower ranking members, etc) all wearing black surcoats and tabards? How difficult and/or expensive was it to really dye clothes black?
Iconographically, yes, the Hospitallers wear black and the Templars wear white (or undyed fabric). This parallels a similar divide among non-military monastic orders. From the mid-12th century, the Benedictines were known as "black monks" and the Cistercians as "white monks" due to the different colors of their habits. It was an important part of the orders' identity.
Black fabric was indeed more costly in the Middle Ages than many other colors, but it's not a straightforward question of dye expense. In terms of dyestuff, cost depended on two things: source of pigment and duration of dyeing process. The most popular option for dying fabric black in the Middle Ages was not, of course, "black pigment." It was to use woad, the bog-common blue pigment, to dye fabric to the darkest possible midnight tint. Then a second dye, typically a red (madder) or yellow (weld), would be added over the top to eliminate the blue tinge.
Woad, weld, and madder were standard dyes, not prohbitively expensive for a decently-endowed monastic order. Benedictine monks could adhere to their Rule not to spend too much money on individual habits.
The cost came in lifespan. The longer a fabric sat in dye, the more it degraded--the shorter its lifespan. Black fabric was more costly due to replacement value.
In terms of sheer cost of dyestuff, for fabrics the grain-based scarlet pigments were the most prestigious, with vermillion being the ultimate achievement of wealth and luxury.
Further reading: To the best of my knowledge, Kenneth Ponting's work on textiles and the textile industry in England is still cited authoritatively for what it covers, although obviously there has been further scholarship since 1980. There is a more recent Cambridge History of Western Textiles which might interest people here for the dyes instead of the knights. ;) This passage in the NCMH on Google Books preview discusses various pigments in the late Middle Ages.
ETA: Our intrepid fashion/textile experts /u/chocolatepot, /u/kittydentures, /u/colevintage might have more information or more recent/accessible sources!