Dain1, good evening.
I am assuming that you have cleaned, as best as possible the beams on the left and the ones on the right still have the black paint?? Cleaning paint from timber above your head is a Headache, not to mention small bits in your eyes??
It looks as if you have differing colours now on the left hand beams? and that finish is not acceptable? probable cause is the treatment of the timber prior to the trusses being fabricated.
Given the differing porosity of the timber once painted, can produce different colours when the paint is removed.
How about you consider shall I call it over-boarding?
Suggest you consider using thin strips of figured Ply wood, this will give you a clean, timber grained surface to work on?? Other thin material can be called into use? Plasterboard, Etc.
Or? There are Proprietary so called "Mock Beams" produced in various timbers, they are designed to wrap around existing down-stand beams to give the impression of old Pre-finished beams, BUT there is generally a BUT?? Cost??
Hopefully some of the above may be worthy of a glance??
Ken.