Fear not with your 1970's council house cement pebble-dash wall fears, it will dry to become white and this is only the first coat of three anyway. It will end up a nice subtle creamy-white like the earth mortar on the left.
Another rainy, windy day so to avoid housework I have started on the render on the back wall because it's inside the hangar and dry. Reasonably dry. Brush down wall, but not too much or there will be no wall left. Spray with hose - using a mister on this wall would take all week due to the amazingly thirsty mud mortar and super porous stone. The key is to experiment with a wee patch first to avoid catastrophe. Hopefully.
Coat no.1 is a mix of NHL 3.5 lime and coarse sand, ratio 1 lime, 2 sand, 0.6 water. It is then rested a bit and then put back in the mixer when more water is added to make a slurry the consistency of cow shit. You then chuck it on the wall using your best tennis forehand technique, remembering to put a hard board below to catch it all when it falls off again. This is normal. Also cover everything with plastic sheet and cardboard as it goes everywhere. Fling it on little by little, and after 3 hours of mixing and chucking and slurping tea you get the above mess. Belters. Do not touch it afterwards - leave it sagging. Mist twice a day for 4 days and protect from sun/wind/rain/cats, then on to the second coat (later). Nothing to it! No need for builders and their short-cuts, mess and expense, no need for cement (very bad stuff). Local, sustainable, vernacular, self-sufficient, and you get big forearms.