FYI if you search for terms like "free vector tartan seamless patterns" you can find a ton of stuff online, and adjust/play with the colours if needed in Illustrator. Waaaay easier that trying to make one from scratch. Or buy credits on
Adobe Stock and buy one you like.
The fantastic site
https://tartanify.com/ compiled 5000 real patterns from the Scottish Register of Tartans, and let you download them in SVG or PNG, as a tile that you can use to make a seamless larger image. (
for the developers/designers you can read about the making of the project on css-tricks)