CSU Extension advises that most established Colorado lawns will have adequate phosphorus (P) and potassium (K) in the soil without additional P and K fertilizer. If you apply a typical blended fertilizer based on N needs, you will usually apply more P and K than most lawns require. 

The Cherry Creek Basin Water Quality Authority and its partners are actively working to reduce phosphorus loading to the Cherry Creek Reservoir and request that you choose phosphorus-free fertilizer. 

For more information from CSU Extension, see: 

Homeowner's Guide to Fertilizing Your Lawn and Garden (XCM-223)

Homeowner's Guide to Protecting Water Quality and the Environment (XCM-223) 

Other resources include: 

Phosphorus-Free Fertilizer Q&A (lovecoloradowater.org)

Water Quality Messaging for Homeowners (CCBWQA)