I wanted to start a new post, since that other one is getting long lol
I don't know which painting specifically was the one you mentioned (the yellow with red and blue strips), because there are a lot that could fit that description lol but I wanted to go on a different tangent about that type of painting (since you asked "why would someone do that?")
So it sounds like it's probably a colour field painting, which was a style that was popularized in New York in the 1940s and 50s, which grew out of European Modernism and Abstract Expressionism (and, for the record, abstract expressionism is one of my least favourite types of art. But that's beside the point lol)
So Modernism is a broad movement, with a lot of different types of art, but one of the most famous painters of that movement was Piet Mondrian, who painted like this:
Composition II in Red, Blue, and Yellow, 1930
He painted a ton of different versions, and his designs have been replicated everywhere. He was also one of the founders of a movement called De Stijl (Dutch for "the style", also known as neoplasticism), and their goal was to attain "pure abstraction".
Essentially, after photography was invented, painting became obsolete as a method of documenting history. Because of that, painters started to explore what painting actually is, as a medium, and test the limits of what they could do with it.
Picasso did abstraction, mostly in the form of cubism, but other artists continued to take abstraction further and further, and Mondrian was one of them. He wanted to completely separate painting from any form of symbolism or connection with reality, so he broke it down into what he believed were the basic elements of painting itself: colour and line.
He composed his paintings using traditional composition methods, but there's no subject whatsoever. It's just an image. But Mondrian didn't get to the level of eliminating brushstrokes, so he never completely removed the hand of the artist from the artwork.
With Colour Field painting, it's a similar concept, but taken even further. It's large blocks of flat colour, and the goal is to make the brush strokes as invisible as humanly possible, so that you can't tell someone actually physically painted it. By doing that, the artist is removed from the art, and all that's left is the colour. "Color is freed from objective context and becomes the subject in itself." (source)
Examples would be Mark Rothko:
And Barnett Newman (who I think might be the painter of the painting you were talking about):
Who’s Afraid of Red Yellow and Blue IV, c. 1969-70
And like, for some size context to his works...
but as for why they specifically headed this direction... colour field painting came out of Abstract Expressionism, and was purposely in opposition to it. Abstract Expressionism was all frenzied movements and chaos, and it was all about expressing angst and other dark emotions. The colour field movement went the exact opposite, wanting clean, flat surfaces, no brushstrokes, and it relied heavily on Gestalt principles (instead of just chaos)
Closely related to that is my personal favourite type of art, which is Geometric Abstraction
One of my personal faves, Peter Halley (who is a current artist):
He uses neon colours and sometimes adds stucco to the paint. I took a lot of inspiration from him when I did my Lego house painting a couple years ago (including adding stucco to the paint lol)
Or Canadian artist Claude Tousignant:
Accelérateur chromatique 90, 1968
His paintings are absolutely massive, and painted by hand. He uses masking tape to make perfect circles (also used that method in my lego painting, inspired by him lol), and there are no visible brush strokes. None.
You don't need to take an art history course. I'll teach you lol
... Actually, in all seriousness, you can absolutely learn everything about art history online. If you search "art history" on YouTube, there's a ton of channels dedicated to it, and several of them are official channels from art museums or art publications.
There's also documentaries on pretty much every art movement and famous artist in history (or just analyzing a specific painting), as well as lectures by art historians... quite often, an art historian will have a particular obsession with one specific artist, and they will spend their career researching them and delivering talks... those are some of the most interesting things to watch, if you find one about an artist you like.
You can also find a lot of videos about art movements outside of Europe, since most of the canon of art history is focused on European art movements, and ignores the rest of the world for the most part. Taking any traditional art history class, you'll still be missing parts, unfortunately. It's basically always goes prehistory to ancient Egypt to ancient Greece to ancient Rome, then continuing in Europe until the 20th century, then moving to the United States. Anything outside of that is either ignored or briefly touched on in any general art history book, unfortunately.
