Yes, you can paint an urban planter box a different color without harming your plants, but the color itself is not the issue. What matters most is the type of paint, how you apply it, and whether the paint comes into contact with soil, roots, or drainage water.
Choose a low-VOC, water-based exterior paint or a plant-safe mineral paint. Avoid oil-based paints, lead-based paints, and products with added mildewcides, fungicides, or strong solvents unless the label specifically says they are safe around plants. For edible plants, be extra cautious: use a food-safe liner or leave the interior unpainted.
Prep also matters. Remove plants if possible. If you cannot remove them, cover the soil and foliage with plastic or a tarp, and paint on a calm, dry day. Clean and lightly sand the surface so the paint adheres well. Apply thin coats, and do not paint over drainage holes. Let the paint cure completely, often for 7 to 14 days, before putting plants back or watering heavily. Wet paint and fumes are more likely to harm plants than fully cured paint.
If you want to paint the inside, line it with heavy plastic, pond liner, or landscape fabric and keep drainage holes open. This creates a barrier between the soil and the paint. Another option is a nested pot system: paint the outer box, then place the actual planted pot inside.
With the right paint, good ventilation, full curing, and a barrier between paint and soil, you can safely give your urban planter box a fresh color. If you are unsure, test a small area first or keep the plants out until the paint is completely dry and odor-free.