At this time of year, it will start getting vaguely light around 5am (assuming clear skies) with sunrise about 5:40am.
If it were me, I'd be on the road every day by 4am. That's about an hour of darkness in the morning, usually the calmest part of the day, both in terms of wind and storms (which tend to build up as the day progresses then taper off at evening).
And, too, this gives a better chance to finish the stage in daylight. While it may be ok descending Rabbit Ears Pass to Steamboat, or Willow Creek Pass to Walden, in darkness (while staying alert for wildlife and other obstacles on the road), wouldn't it be faster and more scenic in daylight?
If your pace is slower, or you're snapping lots of photos, or enjoying a sit-down meal at a cafe en route, you might consider an earlier start. Specifics ...
Saratoga - The first hour towards the Snowy Range is a combination of gentle climb and some upward-trending rollers. By the time you reach the climb proper, it should be light, and by the time you reach the Snowy Range highlands, the sun should be warming you for the descent.
Steamboat Springs - An extended mainly gentle climb through Oak Creek makes for a good start in darkness. It should be light by the time you reach Oak Creek and the store there.
Walden - Thirty miles to Cameron Pass, but most of it in flat to rolling terrain in North Park. By the time you reach the main climb, it should be light. And as with the Snowy Range, you should be facing into the warming sun for your descent. The balancing act with the Poudre Canyon descent is an early descent can be chilly, but the longer you wait, the more time for up-canyon winds (headwinds) to build up, making more work for you in the lower canyon (and more traffic). So generally, earlier is better.