What gives pesto sauce its colour?

The vibrant green color of pesto sauce comes primarily from basil, the key ingredient.

Here's why:

* Chlorophyll: Basil leaves, like all green plants, contain chlorophyll, a pigment responsible for absorbing sunlight for photosynthesis. Chlorophyll gives the leaves their green color.

* Other pigments: While chlorophyll is the dominant pigment, basil also contains other pigments like carotenoids, which contribute a bit of yellow to the mix.

* Processing: When basil is ground into pesto, the chlorophyll is released and the leaves are broken down, further intensifying the green color.

Important note: While the vibrant green color is desirable, it can fade over time due to the chlorophyll breaking down. This is why fresh pesto is best, and why some recipes suggest storing the sauce in the refrigerator with a layer of olive oil on top to protect the color.