Fill a large pot with lightly salted water and bring to a rolling boil. Ease soba noodles into water and return to a boil. Cook uncovered, stirring occasionally, until pasta is tender yet firm to the bite, 5 to 7 minutes. Drain.
Melt butter in a skillet over medium heat. Add onion, green bell pepper, yellow bell pepper, red bell pepper, and snow peas; cook and stir until just tender, 3 to 5 minutes. Add cooked noodles and soy sauce; cook and stir until heated through, 3 to 5 minutes.