It depends on your budget and how much time you want to spend on caring for and watering plants? For high impact, ease of care and lowish budget. Paint the wall with masonry paint. Buy 3 largish tubs. Plant a climber in each like honeysuckle, a small leaved variegated ivy for colour all year and jasmine. Fix trellis along the wall and let the climbers romp away over them. Just feed, tidy up and water every week. Fairy lights along the wall would add a lovely atmosphere.