B767 technician reported during troubleshooting of crew oxygen system; the technician performing the work failed to open oxygen valve to the full open position causing the flight crew to refuse aircraft.

Narrative

3 days prior to the event an oxygen mask box was replaced in Aircraft X. When the aircraft arrived; and pilots were doing their oxygen masks checks; they realized that the pressure dropped 200+psi which is consistent with the crew bottle shutoff valve not being fully open. I am the crew chief of the crew on shift that day; 2 of my colleagues go to the aircraft to start assessing (file numbers were added as additional employees involved in the troubleshooting); that day my crew had less people than originally planned to bolster the hangar crew with our manpower. Also; this callback was made to us when fully boarded with pax and within 10-20 mins to departure time.The 2 employees that assisted had been with the Company for only 1.5 years (new guys but both experienced with local EASA licences). When I arrive to the aircraft; they were bringing steps to access the e/e compartment to check; based on their experience; that the bottle was fully open; which I concurred being the first action to take. I climbed in with the colleague. He removes the inspection wire; I am behind him witnessing; and he checks that the valve was fully open (or at least he thought/felt that it was fully open; we later found out that it wasn't). I know that we have 2 different kinds of bottles; a steel and a composite bottle; I mention that the feeling on the valves are different for each one; but he reiterates me that he felt it was fully open; I didn't question it nor felt it myself. We then proceed to check the masks; and the symptoms were the same; we looked for leaks; checked masks; checked history; checked boxes and everything seemed to be ok. So; we proceed to change the regulator; transducer and bottle assuming we had a bad component. The same person; who never mentioned during the stress of the moment that he couldn't differentiate in between both bottles for being color blind and lack of familiarity with Company components for being new to the company; he is the person installing the bottle; I am again witnessing; he opens to what he thought it was fully open (composite bottle) and then I suggested to back off 1/4 turn as per manual and he backs it to it and then safeties it. We recheck and the symptoms were the same. We then assume it is an indication problem and proceed to apply an MEL in the attempts to save the flight; but pilots were hesitant; and I mentioned that we had a spare aircraft; therefore the pilots felt inclined to take the other aircraft and a rejection to the MEL was made; my crew was at this point on overtime; so we left the aircraft AOG to the graveyard crew to continue the troubleshooting; in my handover to the supervisor I explicitly asked for a fresh pair of eyes to go over our procedures done in the stress of the moment; to start the troubleshooting and correct the discrepancy. I found out the following morning when back on shift that they had found the valve at the resistance point; typical of composite bottles; and still needed more turns to be fully open; that corrected the fault; and the aircraft departed safely that afternoon. I discussed with the guy that did the work with me; and he then said he couldn't tell the differences due to color blindness and lack of familiarity; I went over to explain it and show both bottles in stores and the 3 of us did the training which then was also sent to the entire crew as a reminder.