I've not gone SUL, but I've lived in CO for most of my life and have hiked the Colorado Trail and the CDT. In summer you can basically set your watch to the thunderstorms in the high country. From 1-4pm you can expect to be dodging lightning if above treeline, so you may need to plan your timing across passes accordingly. The temperature typically will hover around freezing at night and in the day the mid 70s once you are in the mountains.
If you have a week and are comfortable with off trail travel, I would recommend hiking from Grand Lake to Silverthorne on the CDT (simply amazing!), or anything in the Weminuche wilderness (CDT route along Creede/Lake City to Wolf Creek Pass above Pagosa Springs especially). For trail hiking, the Colorado Trail from Salida to Creede/Lake City is very easy to follow, challenging and the scenery is hard to beat.
Hope this helps.