It sounds to me like you are working with a Lennox product. If that be so, look on the blower board. You will see ACB Heat, this is your blower speed for heat. You will also see ACB Cool, this is you cooling blower speed. You will also see one marked fan, this is your continuous blower speed. You should also see terminals marked park, These are the blower speeds not being used, these are dummy terminals. The blower speeds should be as follows, Black (high speed) ,Blue (medium high) ,Yellow (medium low) ,Red (low). If you know how to do a temperature rise calculation, you will have no problem deciding what to use where.