During the Vietnam War, amidst carpet bombing and Hamburger Hill, the United States engaged in a particular form of chemical warfare that targeted not people, but plants. Between 1962 and 1971, Air Force transport aircraft were used as souped-up crop dusters to spray ~77 million liters of herbicides (plant-killing compounds) throughout Southeast Asia as part of Operation Ranch Hand. The idea was to destroy both food crops (primarily rice, which is difficult to incinerate) and forest canopy, which North Vietnamese forces took advantage of to conceal their activities.
A collection of herbicides, either single compounds or mixtures, were used by the US in 'Nam. Six of these were assigned a particular color code, a stripe of this color being painted on the 55 gallon drums in which the particular agent was stored. Thus the whole rainbow thing.
Unfortunately, these herbicides weren't just good at killing plants. They also harmed people, in part because they were contaminated with a really nasty poison. It's thought that millions of Vietnamese were directly exposed to herbicides over the course of the spraying campaigns.
Agent Orange is by far the best known of these "rainbow herbicides", largely due to the fact that it accounted for most of the herbicide sprayed in Vietnam. It consisted of a 1:1 mixture of 2,4-dichlorophenoxyacetic acid (2,4-D) and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T). These compounds resemble a group of plant growth hormones known as auxins. They stimulate rapid and uncontrolled cell growth, causing plants to literally grow themselves to death. In lab rodents, 2,4-D doesn't appear to cause cancer but can cause kidney and nerve damage. People who have inhaled this herbicide or ended up with a bunch of it on their skin tended to get an upset stomach and nerve damage affecting their ability to move and feel with their body parts.
Agent Purple, like Agent Orange, was a mixture of 2,4-D and 2,4,5-T. It was only used between 1962 and 1964.
Agent Pink and Agent Green each contained only 2,4,5-T. Boring.
Agents Purple, Pink, Green, and Orange were all contaminated with trace amounts of TCDD, a dioxin. Dioxins, more technically referred to as polychlorinated dibenzodioxins, are a group of incredibly toxic byproducts associated with the production of chlorine-containing herbicides such as 2,4,5-T. TCDD is likely responsible for increased rates of cancer and birth defects in Vietnamese populations who were exposed to this compound via the herbicide sprays. Researchers looking at veterans of the Vietnam War who were exposed to TCDD (via Agent Orange) have found evidence that this exposure is linked to prostate cancer and diabetes.
Agent White was a little different. It consisted of a 4:1 mixture of 2,4-D and picloram, another synthetic auxin herbicide. Since there was no 2,4,5-T included, there was no TCDD. Picloram doesn't appear to be all that toxic, although it does tend to stick around in the environment for a while.
Agent Blue contained cacodylic acid, an arsenic-containing herbicide that is very different from the chlorine-containing synthetic auxin pesticides found in the other five agents. Arsenic compounds, like inorganic arsenic, are able to disrupt energy production in the cells of both plants and animals. For plants, this can lead to, among other things, the interruption of water transport, causing them to dry out and lose their leaves. For people, organ damage can result. Exposure to arsenic also increases the risk of cancer, heart disease, and other chronic illnesses.