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 soft bodied insects which live on the sap of a wide number of plants. You will find them on the underside of leaves, and also clustering around buds and flowers. Aphids are generally greenish but can also be black, orange, or shades of brown.