Many orchids seem to produce scent in distinct ... "wafts". They don't smell all the time but rather in bursts during the time that their pollinators are around. For example, most pure white flowered species such as brassavolas, angraecums, aerangis and neofinetia will only smell in the evening (or at least the scent will be strongest in the evening). This is because they attract moths that fly at night, that is also why they have long nectar spurs (moths have long mouth parts and can reach the nectar at the bottom of the spurs) and are pure white (which is the most visible colour at night as it reflects the most of the poor light that's available at that time). Flowers that are pollinated by bees tend to smell during the day while the bees are active and, although they come in a variety of colours, pink or purple ones are particularly attractive to bees. In general, but not always, flowers that tend to be orange or red and have no scent are pollinated by small nectar eating birds such as humming birds in the New World and sun birds in the Old World. Birds have excellent colour vision but there are only a handful of bird species in the world that have any sense of smell at all, insects on the other hand tend to have very good olfactory senses in their antenna.