I discovered a trick to make tomato soup a brighter red with natural ingredients:
Sauté onions and tomatoes with a red bell pepper in some oil, add salt, then water and simmer until the vegetables are softened. Puré, mash through a sieve -- if you want to get rid of the seeds and the skin -- and dilute to a soup consistency.
You may add tarragon at the beginning, or parsley at the end, but don't, if you want to keep the soup a pure red.
The pepper does the trick. It also adds a delicate sweetness that compensates for the acidity of the tomatoes. Altogether a delicious soup!