The florist is dealing with perishable merchandise that may be brought in from all over the world. If you simply must have flowers that are out of season, you will have to pay more for them. The more freedom you allow your florist to work within seasonal choices, the more likely you can save money. Always have alternative flower choices in mind, or allow your florist to consider alternatives. Bring as much information along with you as you can.
Swatches are also helpful in determining colors.
Manufacturers come up with thousands upon thousands of trade names to describe colors. Also give the florist some room to manipulate. Let your florist work within ranges of colors or combine different colors together to come up with interesting looks. Oh, one last thought. Try to keep their flowers as similar as possible, so that you won't hear about possible hard feelings twenty years after the wedding has been held.