Whisk oil and vinegar in small bowl to blend well. Season with salt and pepper.
Place greens in large bowl. Toss with enough dressing to coat lightly. Divide greens among 4 plates. Arrange smoked salmon slices over greens on each plate, dividing equally. Drizzle more dressing over salmon. Sprinkle salmon with capers. Garnish salads with red onion and red and yellow tomatoes.