Please don’t try to match a Ben Moore color in the Sherwin Williams line of paint choices. Thanks to differences in color blending techniques, you won’t get the shade you expect.  If you want a close match to Horizon in Sherwin Williams color options, I recommend Sherwin Williams Glacier Bay, although it won’t be as blue or cool-toned as Horizon. Behr’s Winds Breath could be another close color option to consider.