How do I paint storage tubs for use as outdoor planters?

When you say storage tub do you mean a 25 gallon plastic tub?

I used a stone finish textured paint from Rustoleum. They have several shades to choose from. I went with one of their gray selections. Love that it leaves what looks like a stone look on the planter. After a few coats I let it dry overnight and then applied several coats of poly to seal that so the weather didn't damage it. But don't forget to put some drainage holes in the bottom. I also added a few inches of gravel at the bottom of the planters to aid drainage.

Krylon spray paint after lightly sanding to rough it up a bit.

I would recommend scuffing with steel wool first, wipe down with denatured alcohol, then spray paint with a plastic formula paint like Krylon.

I use a spray can paint to paint outdoor fixtures. Makes it easy to do and easy clean up, too.

This is fantastic. Great info and how-to. Before and after pictures.

https://www.lemonslavenderandlaundry.com/spray-painting-plastic/

If they are plastic Krylon spray paint for plastics. If they are metal, use Rustoleum auto primer, can get it at Lowes or Home Depot, let dry well then spray paint it the color of your choice in Rustoleum spray paint.

Rustoleum or Krylon brand paints have spray paints for most any surface. You can make your tubs look like they are metal, happy colors, or even a textured finish. Check out one of the big home improvement stores paint aisle.

We paint plastic all the time with spray paint. Just make sure you get one that's made for plastic. Some of them are primer paints in one.

If they are plastic, purchase Krylon paint for plastic, any color you wish and go to work. Follow the instructions on the spray can. Purchase enough to do the whole job so there is no color variation. You can always return the unused cans when your project is done.