Hi Sharon,

While we don't have a list of just succulents that are hardy here (the list would be short!); the succulent plants that are commonly available and typically succeed here can be found on the xeriscaping plants fact sheet, here: https://extension.colostate.edu/topic-areas/yard-garden/xeriscaping-perennials-and-annual-flowers-7-231/

In Castle Rock, Ice plants (Delosperma), Sedum, and Hens and Chicks (Sempervivum) are some of the hardiest options.  

Additionally, your niece could of course also consider hardy cacti, which for whatever reason didn't make the fact sheet "cut".  Some of my favorites include hedgehog cacti, genus Echinocereus.  They don't have the little spines that travel from the plant like the prickly pear varieties, do, though if you don't mind gardening among the spines, several varieties of prickly pears (Opuntia) do well here too. 

To see some of these plants in action or to get more targeted recommendations, I encourage your niece to call or visit the Extension office in Castle Rock and/or to visit the demonstration garden there.

I hope this is helpful!

Cordially,

John