What can I do to make my barrels nicer looking

antique with black or preferably dark brown

Use a little raw umber (a darkish brown-like color) paint to antique them. Use a dab of color on a rag-- gently streak--OR ... maybe even use a hint of gray, blue, green, whatever goes with your color scheme. Make them harmonize with your color scheme ... it's an easy fix! Be careful of splinters in your finger tips!!! Work top down toward floor (following grain line of the wood). Assuming these are wood-- if NOT-- by "streaking" the surface (as above), you can make them LOOK that way ... Work left to right -- dark to light. Thay should end up looking like a Rembrandt painting! GOOD LUCK!

You can stencil them, put bands or stripes on them... paint rims gold or silver... check on Pinterest... for ideas

What are they made out of? If they are plastic you can use them to store water in for watering plants. If you fill they up and let them sit without a cover for a few days a lot of the chemicals in the water will evaporate. If you paint them a dark color they will absorb some of the heat from the sun (if you want that). Painted our huge plastic barrels white and they hold water for our outside plants.

Paint 1” dark brown vertical stripes on them, about 3-4” apart. On every three or four white areas, pick a spot and paint a 4” long oval, then concentric ovals inside. Vary the position of the ovals on the white bars. When the paint is completely dry, thin some brown paint with 1part paint, 2 parts water. Then do a wash with the diluted paint over all the white paint. Surprise yourself with your wood cask. You can add black horizontal stripes for iron staves. Or go forest by painting a little songbird or owl in each knothole. (Or get bird decals for the same purpose.) 😇

have you tried distressing them by sanding off some of the paint?

Thanks everyone.... I have not yet done anything with them yet... they are still white... they are made of cardboard; shipping barrels.