Open your 2D software (Photoshop, Gimp or whatever you like to use) I will use Photos. Create a New document. like so and click on OK You can use the same values as I did (though, not necessary for this tutorial).
Then create 6 quadratic selections and fill each with a different color. When done fill the background with black. And now save your texture in your project folder as a Windows Bitmap (BMP) or JPEG either one will do
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