Fixing green coloration on clapboard siding
If it's only mildew or algae, you can probably just clean it off pretty easily with oxygen bleach (sodium percarbonate). Mix it up with water to the concentration specified by the manufacturer, spray it on the clapboard, wait about 15 minutes, then scrub it with a cleaning brush. 

I'd test this on a small, non-conspicuous area first to make sure it doesn't effect the color - oxygen bleach won't bleach the pigments themselves, but it may remove oxidation or sun damage on the top of the paint. It certainly shouldn't be as noticeable as sanding and repainting the area though.