I had my inside 120 volt blower motor changed awhile back and a bigger motor than the original was installed because it was the only one I could get at that time. The blower runs to fast. I would like to change it out with a smaller HP 4 speed 120 volt motor. I need to know how to properly wire it. Could something be put on the present motor to slow it down?
Also there has been a browish colored material coming out the ceiling outlets. Could the coils be dirty? I took off a couple of the ceiling vents and the pipes are clean inside.

The wiring diagram should be on the motor itself. Here is a universal color code speed wiring diagram for furnace blowers. Yours should be the lower diagrams since your motor probably has a capacitor.

The color near the ceiling could be almost anything but it never hurts to have your heat exchanger in the furnace tested if it is gas or oil operated.

I'm want to lower blower speed on furnace because central air isn't removing humidity from air.on the blower is a strip wired as follows.white wire to com terminal .black wire to high.blue wire to med high. No wire on med low. Red wire to low

The slower you run the blower the MORE humidity you will remove from the air. The reason is the air has more contact time with the cold coil and allows more moisture to be removed. Now with that being said you also have to adjust the speed to allow for proper air movement in the structure. Read the attachment on the previous post of mine to see the wiring diagrams.

NOTE: Your unit is wired to use the high speed for the A/C. You will need to change the black wire to the blue and try that. Disconnect the black wire and tape off if necessary since you do not want both black and blue connected to the terminal at the same time.