Please keep in mind that the color of a rose and its symbolism is a rather subjective thing. Don't assume that the person you give a rose to understands the meaning behind your choice of color.
As an unofficial guideline, many people seem to agree on the following:
Red: Love, Respect, Courage
Deep pink: Thanks
Light pink: "I think you're special", also ympathy
White: "I apologize", Reverence, Humility
Red roses and White roses (or roses that are red and white): Unity, Mending bridges
That said, other meanings have been associated with these colors over the years. For instance, Yellow has been associated with "Loss of Love" so it could be thought of as a "Let's-Just-Be-Friends" color.
[From Taylor's Guide to Roses
, p. 418:] There are a greater number of roses in the pink
and pink blend color classes than any other color...
[From The Rose: An Encyclopedia of North American Roses, Rosarians, and Rose Lore, by Sean McCann, p. 103:] From the Peaceable Kingdom School in Old Washington, Texas, Liz Druitt wrote that as yellow roses stand for the decrease of love, so people often picked on 'Marechal Niel' to sumbolize disappointed hopes.
[From A Family of Roses, by Sam McGredy and Sean Jennett, p. 45:] The gene for true orange was not in any rose... and now there are several oranges and orange scarlets because of the mutation that gave Kordes' 'Independence'...
[From Fifty Favourite Roses, by John Mattock, p. 77:] Until the very beginning of this century, apart from species or near species roses such as R. ecae, R. foetida and R. harisonii ('Harison's Yellow'), there were no bright yellow roses in our gardens. The predominant colours of the roses grown until that time had been mauve, a pruplish red, maroon, lilac, white, and a wide range of pinks, with crimson to be found in some of the China roses. Orange and flame colours, as well as yellow, were absent... it took many years of patient work before the first success, by Joseph Pernet-Ducher of Lyon. To introduce yellow into his breeding lines he used a wild rose from the Middle East, R. foetida 'Persiana' (the 'Persian Yellow'), a double-flowered form of R. foetida, crossing it with a number of hybrid perpetual varieties... In 1900, he put on the market 'Soleil d'Or'... obviously not a true yellow. It was, however, introduced as such and was undoubtedly a tremendous step forward... it's offspring, 'Rayon d'Or' [introduced in 1910] was true golden yellow.
[From Roses: An Illustrated Encyclopaedia & Grower's Handbook of Species Roses, Old Roses & Modern Roses, Shrub Roses & Climbers, by Peter Beales, p. 108:] Orange is a popular colour, but in colder areas roses of this shade are best avoided since they tend to be less hardy than those of other colours.
Flesh. Flesh colour was once the colour of raw meat, not the flesh, say of your inner arm, as it is known today.
Poppy Red A 'poncel' or 'ponceau' in old French means 'Coquelicot'. ie Poppy.
'Ponceau' is an azo dye formulated in the latter half of the 19th century, and is a pinkish purple or purplish pink somewhat similar to 'magenta'.