Too much sun can cause this - they prefer dappled or partial shade. Otherwise, if you have a variety of S. japonica, these particular cultivars do not like alkaline soil and this can show as chlorosis, or yellowing, particularly if the yellowing isn't even, but more mottled. All Skimmia prefer neutral to slightly acid soil, but S. japonica varieties are more sensitive to alkalinity.

As Bamboo has said Skimmia japonica burn yellow in full sun. They grow naturally in Japan in deciduous woodland and forests where they get filtered sun. If you want green leaves then move them somewhere in shade otherwise they look that sickly yellow color.

also give them a dose of Epsom salts. 1 teaspoon in a litre/2 pts of water will help it make more chlorophyll [green pigment]

I've been told that Skimmias prefer acid soil and will go yellow and die if planted in ordinary soil.

It's not true, Arbuthnot - they like neutral to acid soil, many will tolerate some alkalinity, but Skimmia japonica 'reevesiana' is particularly offended by alkaline soil.

Thank you one and all

I rejuvenated a similar state camellia in my sons garden by top dressing with a whole bag of ericaceous compost, or you can feed sequestrene. Leaf mould also helps with soil structure.