I'm not entirely clear jf whether or not you are you suggesting the existence of a khaki frock with yellow facings? I don't think you are: personally I think such a curiosity to be highly improbable.
I should think that there were fairly significant quantities of BNI tunics lying around in stock: the British held on to the vast majority, if not all, the significant logistic facilities and the Bengal Army was a very big force: 70 regiments of regular BNI, each of 1000 men, alone, without even worrying about the so-called 'irregular' contingents, the infantry components of which more often than not also wore red tunics and were to all intents and purposes as 'regular' as the rest. On the other hand, I cannot conceive that it would have been beyond the capacity of Cawnpore's merchants to re-clothe a regiment in the sort of smock clothing which had become fashionable by the autumn of 57, albeit with so many British regiments coming up the lines of communication in the winter 57/spring 58, I suppose it is possible that there was a shortage of suitable cloth, at one or more points. Equally HM 5th, which had come from Mauritius, in lightweight clothing, might just have been desperate to have red woolen cloth, as I understand that 57/58 winter was pretty cold by the average standard and of course everybody else had gone into red for the cold season. I cannot immediately recollect the regiment's movements after coming out of the Alambagh, in March 58, but if you'll permit me the conceit I'd be prepared to wager they were not wearing red inside the Alambagh without me knowing about it. If I had come upon such a reference I would have recorded it in my little black book - I'm off out now but I'll have a look later.
Dr Mike Snook MBE psc