My two experiences in CO late Sept agree with the guys above - first time out reached 84 one day, then two days later was upper 30's up on mountain with spitting snow. Second trip had snow storm the first night there, then great weather rest of week, 26-30 at night then 60-70 during day.
Staying dry is key. Even the one day in the rain/snow shower I only had on one base layer and a thin packable rain gear - stayed plenty warm since I stayed on the move. I have always stayed plenty warm with just a base layer and thin fleece layer on top of that. If I get warm, just stop for a few minutes - it doesn't take long to cool down. Bring rain gear and extra fleece upper layer with you at all times.