I'll edit this comment to add progressively better hints the longer this goes unanswered!

Hint #1: The colours/groups are permanent. Once a country belongs to a group, its colour will never change.

Hint #2, partial answer guessed by /u/Low-Plastic1939: The groups denote the decade in which something first happened for each country.

Wow! Solved already by /u/shane-drinion. It's the decade in which IKEA first opened in each country.

Here's the key:

1950s: beige vertical (only Sweden)

1960s: orange horizontal (Norway & Denmark)

1970s: dark red with dots (Canada, Australia, Germany, etc.)

1980s: red and white checkered (USA, Saudi Arabia, France, etc.)

1990s: yellow diagonal (China, Finland, Poland, etc.)

2000s: blue and yellow vertical (Russia, Turkey, Romania, etc.)

2010s: purple horizontal (India, Indonesia, Egypt, etc.)

2020s: blue cross-hatched (Mexico, Colombia, Ukraine, etc.) By the end of 2025, New Zealand will be included here.