Time and again in life you may have been accosted by some benevolent acquaintance with the dogma "Say it with flowers!" and surely your resultant action would have been based on the tenets of the situation like mood and circumstance.It is no small wonder that the uninitiated, among others, would ponder on such a mantra, seeking to establish "What is the IT that I must say?" and "What is this thing called FLOWER, anyway?"
Regarding the latter question, Encyclopedia Britannica attempts to come to the rescue by defining the flower as "the reproductive portion of any plant in the division Magnoliophata (Angiospermae), commonly called flowering plants or angiosperms". It is also of interest to note that the anatomical structure of a flower consists of a floral axis (a greatly modified stem), essential organs of reproduction (stamens and pistils) and usually accessory organs (sepals, petals) which attract pollinating insects and protects the essential organs.
Flowers symbolize beauty in most civilizations of the world and mere observation would show that these reproductive structures come in a wide range of colors, sizes, forms and anatomical arrangements.
It would be quite useful at this point to examine what some flowers do 'say'.
They can be used to display expressions of affection and endearment, as decorations (whether personal adornment or environmental enhancement), as tokens of respect, as gifts of cheering, as expressions of gratitude, and even along corporate lines of employee/employer recognition. Each type of flower is designated some specific meaning according to its name, and the convention remains true among different colors of the same group.
The aster signifies variety and afterthought, as it begins to bloom when other flowers are scarce. The daisy projects a picture of innocence, purity in thought and loyal love. The delphinium conjures flights of fancy and ardent attachment, while the iris unveils faith, wisdom, valor and promise. If you are secretly in love, the gardenia aptly conveys this sentiment.
The computer buffs among you may readily identify with the forget-me-not, as apart from faithful love and undying hope, it is also representative of, believe it or not, memory. A lily is not always the same as a lily, because the lily is the hallmark of majesty, sincerity and purity, however the lily-of-the-field implies modesty, sweetness and reconciliation (a return to happiness).
Admiration and beauty are the attributes of the heather, the myrtle expresses love, the orchid denotes magnificence and a beautiful lady, while the peony represents a happy marriage and prosperity. On the note of marriage, dependant on your temerity to attain this office, you would do well to note that the snapdragon means presumption and desperation.
Although the adage 'love is blind but friendship is clairvoyant' may be bandied with indiscretion, each varying color among flowers of similar type makes its own bold statement as to its appropriateness. For example, within the carnation family, the red means adoration, the pink stands for rare love, the white implies sweetness and loveliness but the yellow reflects downright rejection and disappointment.
And now unto the undisputed monarch of the flower kingdom, known to all and sundry as the ROSE.
The rose is designated the piece de resistance, outstripping its peers in terms of popularity hands-down. Many composers of sonnets and love songs highly recommend the employment of these flowers to convey that special message of deep amorous intent and uninhibited love. "Compliment what she does, send her roses just because…if it's violins she loves, let them play!" (One Hundred Ways). "Send one your love with a dozen roses, make sure that he/she knows it with a flower from the heart!" (Send one your love).
But hold on a minute.
One should not be lulled into a state of complacency upon hearing 'a rose by any other name is still a rose' and proceed to wildly bestow them upon the object of your adoration, without giving due consideration to color. As to do so would severely limit any opportunity for definitive expression.
Pale-colored roses denote friendship (including those friends that you would like to know better), and as such, you would send the peach of the specimen to your tennis partner and the lavender to the neighbor who keeps your spare house keys. The white ones, as you can well imagine, say, "You are heavenly!" They convey reverence, humility, innocence and purity, secrecy and silence.
Stepping up the intensity, pink roses symbolize grace, gentility and adoration and can be generally used to say, "You are lovely!" - yet from a fine-tuning perspective light pink implies "You are wonderful!" while deep pink says "Thank you!" The quintessential red rose means "I love you!" in addition to representing courage or respect, while the yellow ones stand for unadulterated joy, gladness. Coral or orange roses convey enthusiasm and desire, emphatically indicating "Be mine!"
Please bear in mind that the many flowers mentioned above may be mixed and matched with captivating effect e.g. carnations together with daisies. Take note also that the number of flowers given from within a certain group can be indicative of one's resolve. A single rose denotes simplicity and perpetual love, and while two roses bounded unto a single stem signals an engagement or coming marriage, comfort can be taken in the fact that a dozen roses so bound, does not necessarily indicate a coming marriage with ten ensuing pregnancies.

Armed with the significance of the diversity within this amazing world, one is better equip to succinctly express purposeful intent when purchasing flowers, and this augers well, especially if the receiver is also aware of these distinguishing nuances.