Flowers are a universal symbol of beauty. To discern a particular meaning, the color of the flowers may be as significant as the flowers themselves. This is especially true for flowers of unusual or nonsensical colors such as green roses. In a dream it would not be surprising to get green roses from a dear friend. This would happen if you are dating your friend's heartthrob in waking life or you sense jealousy from them concerning your romantic attachments.
Consequently you may have acquired knowledge about flower meanings that your subconscious is now accessing to illustrate a point. This can be especially true if you are given flowers by or are giving flowers to another.
Do particular flowers have special memories for you due to childhood, the death of a loved one, or a prom date or wedding?
Here are some common reference points for particular flowers:
Lilac -- Poison, Illness, Death
Daisy -- Indecision about feelings, giver illustrates issue of concern
Orchid -- Sexuality and sensuality
Rose -- Poison White = purity, Black = death
Lily -- Renewal, springtime, ressurection
Narcissus -- Self-love, personal reflection