There are multiple metric units to choose from! This tutorial goes through the process of choosing the best metric unit to use for measuring the mass of a given object.
The most common aphids on houseplants are the light green ones (pear aphids), but aphids can also be found colored pink, white, grey and black. Additionally, winged aphids can appear when colonies are established and fly to infect new plants. Juvenile aphids (nymphs) look like smaller versions of the adults.
Most aphids are naked, without a hard exoskeleton, but some species have a soft cottony substance over their bodies. They can be just about any color. Some have wings and some do not. About one-tenth of an inch long, aphids have long hypodermic needle-like mouth parts, adapted to pierce and suck out plant juices.