Hi Mark,

The two recommended ways are foliar sprays and chelated products that you can apply to the soil.  Foliar sprays typically act more quickly, within a matter of days, but could coverage is essential; anywhere that isn't hit with the spray will show symptoms. 

Chelated iron products applied to the soil are more common, especially for large trees.  In almost all Colorado soils, the chelate EDDHA is the only one that will work well--high soil pH limits the utility of several other chelating agents, including EDTA.

Working to improve soil tilth (over decades) by adding organic matter and/or sulfur can gradually lessen the need for iron supplements, but many trees will need them for life.  If you're in the position to choose a new tree, pick one that is tolerant of high soil pH in order to avoid the problem.

This section of the Colorado Master Gardener training curriculum covers some details of the condition of iron chlorosis and thoroughly reviews treatment; you may find it useful: https://cmg.extension.colostate.edu/Gardennotes/223.pdf

I hope this is helpful!

Cordially,

John