Per the link

Historically it has been the policy of the Food and Drug Administration to allow the artificial coloring of the skins of mature oranges. It is a common practice to color the skins of oranges in certain orange growing areas of the country because of climatic or cultural conditions which cause the oranges to mature while still green in color. The coloring of the skins of oranges is done in one of two ways:

Adding a color to the skin of the orange, Citrus Red No. 2 (21 CFR 74.302©) if they are not intended or used for processing and they meet the maturity standards for the states in which they are grown.

Subjecting the orange to ethylene gas by a commercial process. This hastens the blanching process which apparently takes place normally after picking.

Terry O’Reilly’s radio show / podcast - ‘The Age of Persuasion’ … had an episode where he mentioned the whole marketing of oranges from the early 1920s and 30s

The whole tradition of drinking orange juice in the morning? … it was a marketing invention created by some marketing geniuses who were looking for ways to sell more oranges. They first marketed the idea of getting people to eat an orange for breakfast because it was a healthy thing to do. Unfortunately, it didn’t sell that many oranges. Then they discovered that it was more lucrative to get people to drink orange juice in the morning because it takes about three or four squeezed oranges to make a glass of orange juice. By encouraging people to drink orange juice with their breakfast, they figured out how to sell many more oranges to people across North America.

Then they got to the point of growing oranges just to juice them. Unfortunately, when you mass render oranges into a liquid, it doesn’t look orangy enough … it comes out like a dull grey/orange/brown. They also remove just about all the water from the liquid so that they can store thousands of gallons of this stuff for future sales. Then when its ready to be used, they just add water and its ready to go. It still tastes good, it just doesn’t look good … so they add orange colouring to it.

It’s amazing just how many common things we have in our lives that actually come from some dumb marketing trick to get us all to buy things to make some company and their share holders money.