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.
Aphids are very common in most home gardens, depending on what part of the country you live in. I know here in Virginia, aphids are a dime a dozen. Here's a little information on what an aphid is, what kinds of damage they do in the vegetable garden and tactics you can use for getting rid of aphids.