Three ways to get it right
Match it. Pick flowers or ribbon in the same color family as the dress. This works best with soft colors like blush, lavender or light blue. With bright colors, an exact match can be hard to find in real flowers.
Complement it. Choose white or cream flowers with a ribbon in the dress color. This is the safest choice and looks good with almost anything. It also works if you only know the dress color, not the exact shade.
Contrast it. For bold dresses like red, black or emerald, white or blush flowers can stand out beautifully. Silver or gold accents work well with sequins. Show the florist a photo of the dress if you can.