Harman Direct Positive Paper is orthochromatic with some sensitivity in the yellow part of the spectrum.
I use a orange filter when taking portraits with this material. This avoids the rather ugly rendition of skin tones that orthochromatic materials tend to deliver. The orange filter I use costs 1.5 stops on panchromatic film but a lot more on Harman DPP. Before exposing Harman DPP I preflash by giving 1 lux.second worth of tungsten light from an enlarger.
In my skylight studio I set Harman DPP at EI = 0.6 on an incident light meter and shoot with more hope than confidence. So far the results are ok but I suspect that other lighting systems, flash or tungsten, would demand different EI values.