Preheat the oven to 350°F (175°C). Line a baking sheet with foil and arrange the tostada shells in a single layer; set aside.
Spray a large nonstick skillet with cooking spray and heat over medium.
Add the ground chicken to the skillet and cook, breaking it apart with a spatula, until fully cooked through.
Sprinkle the taco seasoning and ranch seasoning over the cooked chicken, stir to combine, then remove the skillet from the heat.
Divide half of the grated Colby Jack cheese among the tostada shells, spreading it on the bottom of each shell.
Top each shell evenly with the seasoned ground chicken, then sprinkle the remaining cheese over the top.
Bake the assembled tostadas in the preheated oven for 10 minutes, until the cheese is melted.
Remove the tostadas from the oven and top each with shredded lettuce, diced tomato, 1 tablespoon of the cilantro avocado yogurt dressing, and chopped cilantro.
Serve immediately.
