this is what i had around me, since then i’ve learned that you can buy pigments to color stuff, of course, but this way worked too

starting with sawdust and food coloring, mix some color into the wood

in the photo below you can see that the largest pieces of wood don’t absorb the color

mix with epoxy:

drill a hole and insert a magnet, maybe using regular epoxy to fix it in place, then putting the colored sawdust epoxy mixture over the top

let it dry for 8 hours or 1 day. if the epoxy isn’t dry it won’t sand away nicely, it will kind of melt away and make a sticky mess instead.