A bush (or vase) of white roses is made to turn red.
The roses have been well dusted with aniline red powder, and shaken so that the excess dust is removed. Upon application of a spray of alcohol or cologne, the chemical turns red.
The aniline red is actually white, otherwise this wouldn't be much of a trick. It is pretty slight anyway — that's why it's recommended that this be an at-home party performance.