Keep brand colors

Open Theme settings and use the Theme randomizer.

Enable Primary or Secondary to preserve that color during generation. Select the color swatch to open the picker and choose a palette color or enter a color value.

The Palette tab offers preset colors; Picker offers adjustable color controls. Check the displayed contrast feedback and inspect the actual text on both previews. Close the picker when finished, and leave the brand-color checkbox enabled if subsequent generations should preserve the selection.