Add half of the onion to the pan and cook for 2 minutes.
Add half of the garlic and all the bell peppers to the onion mixture in the pan. Continue to cook the vegetables in the pan and add all of the yellow squash and zucchini to the pan.
Lightly season the cooking vegetables with salt and pepper.
Let the vegetables cook until the squash is just crisp-tender (about two minutes).
Remove the vegetables from the pan into a large serving bowl.