Spotted yellow, as in round dots, or yellow patches? Yellow in between veins only, or generalised? Where are the plants growing, sun or shade, soil or pot? If in pots, how long have they been in the same pots, and did you use ericaceous compost?

Skimmias are best in a generally shady spot, as the hot sun can scorch the leaves. They don't like heavy wet ground, or ground that's too chalky (they will stand quite alkaline conditions, all other things being equal). Is it just the oldest leaves, because they will fall off anyway, naturally being shed at this time of year, or a little earlier, as the new growth ripens/matures. Phil J