The answer, I think, is not to put too much green and red or green and orange into a soup which, as you'll remember from nursery school, makes brown. The proportions I used - 1 onion, 1 carrot and two sticks of celery - along with around 350g of cooked parsnip, I would guess (about 1 1/2 cooked parsnips) is just about right.
You also need something to contrast with the uniform smooth texture of a liquidised soup. I hit on the idea of a slick of garlic-flavoured raita - plain or, as it happens, soy yoghurt mixed with crushed garlic and salt which was really good. If I'd had some fresh coriander or parsley I'd have added a scattering of that. Or some roasted whole cumin seeds, come to think of it. With some warm naan or pitta bread it would have been a real feast. But ordinary bread was just fine