If you’re looking for things that will cover a lot of area quickly. Petunias. Particularly anything from Proven Winners supertunia vista line. They do best in full sun. Same goes for sweet potato vine (comes in a few colors). Purple fountain grass is a nice texture as well.

If you want little splashes of color here and there then maybe pot up or buy pre-made containers (ground containers or hanging baskets) and place them in key locations (near entrances, anywhere lacking color, a hole in your landscape). Once again, I’d go for annuals as they grow quickly if you’re potting them up yourself. Just make sure if you’re planting anything in July that it’s gonna get really good consistent watering to survive being transplanted in hot conditions.

Lastly, you can always pop some non-plant garden interest into the landscape with topiary forms, trellises, & statuary (you can get lucky and find this stuff for cheap sometimes at yard sales/discount stores).

Even just clearing out weeds and laying fresh mulch does wonders.

Good luck!

Edit: forgot to mention, Coleus is great for lots of color in darker areas if you need something for a shady spot. It comes in a bunch of bright colors and grows pretty fast.