Which variety of Pittosporum tenuifolium? Some of the cultivars have yellowish green leaves anyway. Assuming you mean they've changed colour to yellow, then give a broad spectrum feed (over here, that would be Growmore granules) and keep well watered. Manure will feed the soil, so won't help the plant quickly enough if its short of nutrients, but you can add manure at the same time as Growmore or similar - if you're using something like chicken manure pellets from a box, these should be put into water for a few hours before digging into the soil round the roots. Note this plant prefers semi shade.

Thanks for your help, I shall give that a try, the garden they are situated in has full sun most of the day and are over 2meters. Also a thick layer of the chunky mulch had been put on it about 6months ago.That maybe preventing the nutrients from getting to the roots.