From what I know, it has to do with the word red covering the purple part of the spectrum long ago, and the "need" for the word purple came out later but purple things that were named red didn't get renamed.

You can look at japanese right now and 青い covers both blue and green, with a relatively recent addition of 緑 to cover only green (thus possibly pushing 青い in the future to be only blue, who knows)

Because they are only purple on the inside. Do you think apples are white? Or bananas?

in hungarian it's purple onion, you just choose the wrong language :)

why do we call green grapes white

So, this is kinda weird, but general colors didn't have names for a long time. Like in Greek texts by Homer he describes the water color as dark wine color because the name "blue" wasn't a word. So it's possible when they named a "red onion" purple wasn't a color and considered a shade of red.

Also as others have said purple was the color of royalty and they didn't want an association.

Because the dry outer skin is a rich red colour

In Spanish we actually call them "cebolla morada" that means purple onion :D

It refers to the communism of the onions

What really gets me is that the compounds responsible for this hue in food are called "anthocyanins" which means "blue flower." I can't even.

It wanted to be associated with the proletariat not the ruling class.

We call them purple in Spanish... English has brain damage as per usual.

Jess and Rob on Words Unraveled talk about color word origins, and their discussion is illuminating about the cultural variance in perception of color. Now I am at peace with all the seemingly weird color names for things.

For a quicker tour, Vox talks about the order that languages fill in color words as cultures begin to refine their distinction.

I'm crying

There's a joke in Bulgarian somewhat related to that. What's commonly referred to as a prune plum we call something that roughly translates to "blue plum". The joke goes something like this:

-- Dad, what's this?

-- It's a blue plum.

-- Why is it red?

-- Because it's green.

Language is weird, but it's ironic that our use of it makes it so.

I spent way too much time sitting in the backseat looking out the window on long family drives wondering why the fuck they kept referring to the orange cheddar cheese as "yellow" cheese. Still kind of annoys me today after decades of getting older and settling down a bit.

The skin of red onions is reddish to purple. The flesh is purple much like shallots. But the skin sets them apart.

The inside is purple, but the outside is red, especially if it's dry.

McCarthyism

I've found the color in a things name usually points to what color dye you can extract from it.

Calm down... calm down... Perhaps consider putting the onions aside and trying a nice parsnip.

Same reason they call the tree a "Redbud tree."

Barely literate settlers were colonizing and writing home about the beautiful Puple Prule Purlple Lavendaar Red-Bud Trees.