Have a 5-2 programmable Honeywell thermostat - replacing my old mercury unit. Have a spilt-system Bryant heat pump.
Brown - Br
Green - G
White - W1
Blue - B
Yellow - Y
Red - R

Connected G, W1, Y and R (not B, not Br). Heat works fine. Just switched to cooling and I only get hot air. Any ideas?

When you switched to cooling did you condensing unit start to run? One problem could be that there was a problem before you switched the thermostat. Like it could be low on freon or the contactor is not putting power to the compressor. These are just thoughts.

If it is in the wiring of the thermostat, Did you connect the same wires that you took off the old one. And another question why did you not use the brown wire. You can look at the circuit board and see if the brown wire is just an extra wire or maybe it should be connected to something.