If you were a close friend of the deceased, perhaps you may want to choose flowers that he/she loved. You can also choose more understated colour variants of those flowers. However, as a general guide, here are some flowers that are suitable for conveying your condolences.
White lilies are associated with heaven, purity of the soul and rebirth.
Carnations could be white (pure love and innocence), red (admiration) or pink (remembrance).
Chrysanthemums convey death and mourning.
For a wider range of colours, go for Roses. White roses for purity and innocence; red for respect, love and courage; pink for love and grace; crimson for grief and sorrow; and yellow for friendship
Orchids are also a good option. Usually white Phalaenopsis and Dendrobium orchids are used to represent sympathy and eternal life.