Bring stock to a boil in a small saucepan. Add saffron; stir to dissolve. Turn off heat; let infuse while proceeding.
Heat butter in a large saucepan over medium-low heat. Add onion and garlic; cook until they begin to soften, about 4 minutes. Add celery and bell peppers; cook, covered, stirring occasionally, until softened, 18 to 20 minutes.
Add wine, and cook, stirring occasionally, until liquid has evaporated. Add infused chicken stock, salt, and cumin. Bring to a simmer, and cook until vegetables are very tender, about 10 minutes.
Let soup cool slightly; puree, in batches if necessary, in a food processor or blender (do not fill blender more than halfway). Return to clean saucepan; stir in half-and-half or milk, and then reheat gently (do not boil; the mixture will curdle). Season with black pepper and cayenne to taste. Serve hot or chilled, garnished with sour cream.