Alumilite Liquid Color Dye (Brown, 1 oz) Highly Concentrated Tint for Casting Resin, Polyester, and Urethane - Ideal for Clear, Translucent, or Opaque DIY Arts and Crafts, Beginners & Professionals

You can find brown grout anywhere but you can also mix colorant. Iron oxide powder comes in lots of color and binds with cementous grouts (not mica powder because it won't bind with the cement) but epoxy grout mica powder works amazing and is used specifically to color epoxy based products. It also comes in infinite colors including metallic. You can also use liquid dyes Brown iron oxide powder - cement based grouts only (though you may as well just get brown grout Bronze metallic mica powder - for coloring epoxy grout. Have used it many times (in copper color) and it looked amazing Alumilite bronze dye - a little goes a very long way.