If they are a light gray-green, especially along the veins, that may be natural to that variety. Original A. spinosus is silvery gray, and its more common hybrids are varying shades of gray green to bright green. If you are gardening in southern France, and the leaves are turning yellow-green, it may be that they are going to sleep for the summer, and will regrow in fall. In the north, that's more likely to be a nutrient deficiency, probably nitrogen.