Why is mucuous a different colour when we're sick?

[Mr Rayon]

When we have just become infected with the common cold why does our mucous change in colour and texture. The colour changes from a more transparent clear colour to a thicker yellow-green colour. Why is this? And how does this happen?

 

Anyone know?

[swansont]

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Moderator Note

This is a medical/physiological question. Ask it in the appropriate forum, please. You've been asked to post in the proper areas way too many times, and had mods move threads for you as a result. You need to start doing this yourself if you wish to participate.