I'm trying to change the color of a flower pot in a picture, as well as the leaves in the picture. I'm working with paint.net (but am open to any other free programs). Here's a little bit of the picture:
There are tutorials on YouTube about changing colors in photos. Most demonstrate the techniques with Photoshop, but most image editors that support layers can be used. Generally, the methods involve creating new layers and layer masks, along with:
Changing the blending mode to color, overlay, or soft light.
Changing the hue.
Here are my results using GIMP. Adjusted levels first. Then changed hue for the blue pot, and used overlay blending for the green pot.