PE, not to argue, but the recent work on cyanotype rex has demonstrated in-camera negatives, cyanotype-to-cyanotype contact printing (through heavy paper), and exposure of prints with ordinary tungsten light. The very same chemicals are vastly faster (based on my reading, it looks like several stops faster) when the sensitive iron and the colorant are separated; my hypothesis (as a non-chemist) is that light energy goes to reducing ferric to ferrous without being soaked up in the secondary reaction or absorbed by its products -- which might reasonably mean both less energetic (i.e. longer wave) light can do the job, and a great deal less of it is needed.

Of course, "in camera" is subject to interpretation, but I take it as meaning it's practical to make (possibly multi-minute) exposures in camera and produce usable negatives which can then be contact printed back to cyanotype for a positive. Even if the speed is more commensurate with Becquerel process Daguerreotypes than, say, collodion wet plate, it'd still be a useful alternative for some subjects and in some circumstances.

HTML, I think you need *green* ferric ammonium citrate, same as for cyanotype. Mind you, I have no real idea what the difference is between the two, but everything I've ever seen (photographically) that called for ferric ammonium citrate specified the green. I haven't had a chance to try a rex-type cyanotype, but hope to do so in the next week or two (temporary change in schedule will reduce my time conflicts with my darkroom doing double duty as my wife's bathroom). Unlike others who've posted the prints here on APUG, I've attended no workshops, and been able to find no detailed information on line; haven't even been able to read Herschel's original work describing the process, so I'll be more or less reinventing it. Should be interesting. I might try developing one in strong tea if I get good results with the regular blue...