Thinks of how to look up some data on this.

There are a bunch of "by decade" style things. I bet someone has done one for interior design.

searches

If one takes this as authoritative, I'd say that it well predates Gen Y. Going by the text in this article, it sounds like the peak was maybe the 1960s or 1970s.

https://mcarthurhomes.com/design-by-decade/

1960s

Bright colors were introduced in the 60s, along with new shapes and designs. Neon colors, bold prints, and texture fabrics were common design elements of home interiors. Couches and chairs were often covered with wild fabrics (influences of the hippie movement). 60s homes also had shag carpeting and wood-paneled walls to create a cozy space. Lamps were also a huge part of defining any space; lamps in the 60s had interesting shapes, and paper lampshades were colorful.

Influenced by the space race, furniture and decorations in 60s homes looked like they came straight from the future. Homebuilders also integrated technology and used new materials in home construction.

1970s

Color continued to be a large part of interior design in the 1970s as self-expression flourished. Mustard yellow was a favorite, but it was often combined with other bright colors to create a unique palette. Crafts and DIY projects were commonly seen throughout the 70s house to reflect the individuality of the time period.

Nature replaced the technology influences from the 60s, as society became more socially conscious. As a result, design relied on organic lines, colors, and forms. Many homes had large windows or skylights to bring in natural light. And interior designers also began to think more about self-sufficient designs and creating an environmentally-friendly space.

This says that pastels started to become a thing in 1980s

that's still color, but more muted.

1980s

Triangles were a popular shape in the 1980s, and pastel colors were extremely popular. Pastels offered a softer color palette than the 70s. Floral patterns were popular, especially for drapes and bedding. Chintz often decorated homes in the 80s, along with floor-sweeping window drapes.

Overall, interior design trends of this time period were often a conglomeration of previous decades. Interior design in the 80s reflected opulence, ornamentation, and over-indulgence. But unique to the 80s was the concept of an open kitchen. This layout was designed for family living and helped create a more open dining area and living room.

In the 1990s, they say it started to be common to just have beige or white rooms:

1990s

During the 90s, interior design focused on creating a comfortable space. Homes had neutral colors and featured natural wood. Natural wood cabinets were extremely popular in kitchens. If a kitchen didn’t have natural wood cabinets, it probably had white cabinets. Walls in some rooms were covered in a patterned wallpaper to add some color and texture, but it wasn’t uncommon to see a room that was just beige and white.

Furniture and decor were somewhat minimal, especially compared to the 80s. Wall-to-wall carpet helped some homes feel cozier. And long drapes continued to be quite popular.

They don't say much about the 2000s except that green became more common as an accent:

2000s

Interior design in the 2000s was extremely environmentally-friendly. Greens became more stylish as a wall color or accent color. Homeowners sought after materials that were better for the environment, like recycled materials, and environmentally-friendly appliances. Some homeowners went as far as to bring the outdoors inside by building sunrooms to let in a lot of natural sunlight.

Overall, the layout of homes in the 2000s was more relaxed than in previous decades. Open floor plans allowed interior designers and homeowners to choose what they wanted a space to become. For example, it was common to have an “office” area with a computer in the heart of the home instead of in a separate room.

They say that neutral colors dominated in the 2010s:

2010s

Because of the global marketplace that exists today, anyone can be an interior designer. Cookie-cutter designs just don’t cut it. Homeowners looking for something unique to make their house stand out. Designs today feature handmade craftsmanship and showcase the DIY attitude. And you will find eclectic decor that makes a home feel authentic and genuine.

While there is still room for accents of color and sections of prints, homes are typically painted with more neutral colors like grays and tans to create a clean look and a cozy atmosphere. Homes continue to be built with open layouts to accommodate families.

And that you still had neutral tones in the 2020s (and minimalism is mentioned)

2020s

The 2020s interior design trends focus on sustainability and smart home technology, reflecting a global awareness of environmental impact and technological integration.

Eco-friendly materials and upcycled furnishings dominate, supporting a greener lifestyle. The rise of smart devices and home automation allows for energy-efficient and convenient living. Style-wise, there’s a shift towards minimalism with a twist, incorporating bold statement pieces and vibrant accents against a backdrop of neutral tones.

Multifunctional spaces are essential, accommodating remote work and wellness activities. Natural light and biophilic design principles enhance the connection to the outdoors, promoting health and well-being in home environments.

So, this doesn't have numbers, and I like numbers, but I'll buy that it's at least a rough approximation of what things looked like. If this is correct, the shift away from vibrant colors had to have at least started by the 1980s. If you figure that new households had maybe people that were, say, 20, that'd be people maybe born in 1960, so late Boomers or maybe early Gen X are about the latest that the shift could have started.

That being said, it also seems fair to say that it was a progression, and it was probably taken further by later generations.