Tulips are considered spring bulbs because they bloom in the spring and have true bulbs as their underground root structures. Daffodils are also plants with true bulbs; they're planted at the same time as tulips.
Many tulip varieties grow well in Colorado, including Lilac Wonder, Red Riding Hood, Maytime and White Emperor. Choose the largest bulbs for larger spring flowers the first year. Expect bulbs labeled as "early blooming" to be smaller than other tulip bulbs.
Fall is the ideal time to plant tulip bulbs. According to the Colorado State University Cooperative Extension, planting Colorado tulips in September will produce the healthiest blooms in the spring. You can always plant them later in the fall -- as long as the ground is still workable -- but the bulbs may be damaged from store or nursery storage by then.