There are several different hacks to try to keep your basil pesto bright green in color. 1. Blanch the fresh basil leaves for 3 seconds and then plunge into an iced cold bath. Wring to remove water and proceed with pesto recipe. 2. Freeze the blades of the blender or food processor. This keeps the basil from getting warm and thus reduces the oxidation. 3. Avoid over processing the basil leaves. Only blend the pesto for short bursts to prevent the motor on the food processor from getting warm. We have better luck keeping our pesto bright green with our high-speed blender. 4. Add a layer of extra virgin olive oil on top of the fresh basil pesto to prevent the basil leaves from oxidizing. 5. Refrigerate the pesto promptly after blending if you are not going to use it right away.