How does a lens affect the saturation of a picture?

In many reviews people describe how a lens can affect the saturation of a picture, especially when it comes down to a comparison of two similar lenses, e.g. 50mm 1.8 vs. 50mm 1.4 or something like that and they point out that one lens is delivering stronger colors than the other one.

What is responsible in a lens to affect the strength of a color?

Answer

The quality and makeup of the lens elements used in a camera lens can have an effect on transmission. Top-notch glass will usually have high transmission, allowing through as much visible light as possible while filtering as little as possible. However, top-notch glass will also usually have coatings, usually multi-coating, that will have its own effect on transmission, and may introduce color cast or affect flaring. Glass that is not coated or cheaply coated will usually exhibit more flaring, which can have a significant impact on scene contrast.

Cheap glass may not have a high transmission index, in which case for a given exposure value, the overall contrast may be lower than quality glass that has a higher transmission index. Contrast affects both luminance as well as color saturation, and lower overall scene contrast in a color photo will usually also affect the apparent saturation of color. Cheaper glass or cheaper lenses may also exhibit various optical aberrations, many of which are explicitly related to the convergence or divergence in how different wavelengths of light focus. This can create purple/green fringing as a scenes depth progresses and focus changes, which can also have an effect on color and saturation.

Higher quality lenses will usually have better control of flare, higher transmission glass, better multi-coating that has a minimal impact on transmitted light, fewer optical aberrations, etc. etc. This all affects the ultimate quality of the images produced, and is most visible in contrast (both overall scene and micro contrast, the latter being an effect of lens resolution) and color saturation. Thus is the benefit of a high quality, more expensive lens...you really do get what you pay for.