Purple sage can be a straight substitute for Common Sage especially as it loses colour when cooked. The taste is the same... It is more decorative in the garden however and makes an excellent addition to a mixed herbaceous border where its gentle purple foliage brings structure and colour from spring to autumn. The leaves are an oval shape with that unmistakable perfume so often found in Italian cooking. And the flowers are a mauve blue and so plentiful that the whole plant can be covered.
If you plant purple sage with other perennial herbs such as rosemary and oregano you create a near instant Mediterranean touch that looks good all year and that will supply key fresh herbs for the kitchen.
You can harvest leaves all year but for soft, tender leaves in the winter, cover a plant with fleece. You can also grow purple sage in pots where it enoys the good drainage. Feed after flowering and don't overwater. Like Lavender, Sage does become woody eventually, even with the best care, so it is a good idea to replace them every 5-6 years. Ig you grow common sage as well, then you can stagger the replacement so you never run out.
Sage is also an important ingredient in a multitude of Italian offerings, probably because it is one of the best herbs to counter the fattiness of many meat - think osso buco - and nut recipes.