Colour

Red: love. They can also carry connotations of respect, and a promise of courage for trials to be faced in the future.

Pink: admiration and love in its earliest stages. A very pale pink, particularly in a carnation, can mean sympathy, whereas the darker shades tend to imply gracefulness and the admission that one has 'fallen in love.'

Yellow: joy. They can also mean a generally happy, excitable person who is well-loved, although some books record this rose as a rose of unsteady affections.

White: innocence. They are also associated with humility, awe of the lover, a respect for the purity of the lover and is also a rose of secrecy and silently recognised affection (they could therefore be used as anonymous deliveries to an office or workplace).

Orange: enthusiasm and lust for life. An energetic or eccentric lover would probably appreciate the vibrant colours, as this is the sort of personality denoted by the shade.

Dark red or burgundy: passion and recognition of intensity and beauty.

Personal Attachments
Some flowers that should not be given to a lover unless there is a personal attachment to the species:

Begonia: 'Be careful of my affections'
Purple Carnation: Capriciousness
Striped Carnation: a refusal of a lover's attentions
Yellow Carnation: rejection
Cyclamen: goodbye forever
Geranium: 'I am making a mistake'
Hydrangea: heartlessness
Larkspur (pink): fickleness
Lotus: an estranged lover
Marigold: Jealousy, grief
Snapdragon: an admission of deceit

Size and Number of Roses

Rosebuds are generally given to younger, or more innocent lovers. They can also be used as the first confession of love, although white rosebuds indicate an affection that the suitor cannot declare openly because the lover is too young or innocent.
A single rose indicates the simple message 'I love you.'
A bunch of roses represents gratitude or great admiration for the recipient.
Two roses with their stems twisted together suggest engagement; this symbol was sometimes used as a subtle hint before a proposal of marriage.
A thornless rose indicates love at first sight.