Here's another recipe for colored cauliflower. But if what you have is white cauliflower and white potatoes, they will work just fine and the soup will be delicious--just not purple. If you're counting calories, you can use skim or low fat milk.
In a medium saucepan, melt butter over medium-low heat, add leeks and cook until soft, about 5 minutes.
Add cauliflower, potatoes, milk and salt and bring to a simmer.
Cover and simmer on low heat until vegetables are soft, about 25 minutes. Do not boil.
Remove the vegetables from the heat, let cool slightly and purée them in a blender, with an immersion blender or in a food mill. (If using an immersion blender, cover with a towel to avoid splattering.).
If serving warm, reheat gently and serve with a drizzle of walnut oil.
If serving cold, chill in the refrigerator before serving (also with walnut oil).