If you're considering the Colorado Rockies area (geography, not baseball ;) ), definitely look to later in spring. April is not a good time to be here for camping purposes. April is among the snowiest months in Colorado, and snow in May, though not common, is not out of the question. Most campgrounds don't open until May, and in the high country, it's mud season...that transition from lots of snow to not so much, in April and May and even into June. Many places don't fully open until Memorial Day, such as Trail Ridge Rd in RMNP, Mt Evans Rd, and some of the seasonal high passes. If you can wait until Memorial Day passes, the wait is worth it.
Also, since you've posted your message in this particular forum topic, I'm presuming that amenities are not a high priority, but scenery is. The need for FHU are less than the need for a quality experience, yes? If that's the case, then definitely waiting would yield a vast opportunity in such places as Taylor Park and the Arkansas River Valley and Collegiate Range, the mountainous southwest part of Colorado around Durango/Cortez/Ouray/Silverton/Pagosa Springs, the mountain areas around RMNP including Poudre Canyon, Red Feather Lakes, Grand Lake/Granby/Estes Park, the resort areas of the central mountains...there's much to see and do in Colorado!