Geometric Triangle Pattern in Adobe Illustrator

Way 1: vary_hues.js 3rd party script

1. Create Polygon  and rotate it with Shift. Use only stroke with no color fill.
2. Using Line segment tool (Smart Guides under the View tab should be enabled) create 3 lines to connect all corners from each opposite sides.
3. Select all and press Divide option under Pathfinder.
4. Duplicate objects to the bottom with overlay.
5. Duplicate the column to the right with no overlay.
6. Select all and fill, then disable stroke.
7. Download the script (link to download) and move js file to the scripts folder (e.g., AI > Presets > en_US > Scripts), save the project and restart Adobe Illustrator to be able to see the script in UI.
8. Select all filled objects and go to the File > Scripts > vary_hue, then type any number, e.g. 20. If we use less number – then less color changes will applies, if it will be around 90, then it result in multicolored pattern.

Way 2: RandomSwatchesFill.js 3rd party script

Steps 1-7 the same as above using this link to download.

Step 8. Select all filled objects and select Swatch you want to apply for the pattern, then press to the File > Scripts > RandomSwatchesFill and this script will be applied.