Pale iceberg lettuce, for example, has only a fraction of the nutrients found in darker romaine lettuce. And within a single head of lettuce, the darker outer leaves, which are exposed to more sunlight, are more nutritious than the paler inner ones. Specifically, darker green lettuce leaves are likely to be significantly higher in beta carotene, folate, vitamins C and K, calcium, magnesium, potassium, and iron.
The thicker, whiter part of the lettuce leaf is also much higher in water. It might be time to start dividing those lettuce leaves more equitably between your two salads! Alternatively, pump up the nutrition in your salad by adding other veggies, such as carrots, bell peppers, and tomatoes.