Letro turning a dark tint?shelf life?

[skinnykenney]

How long is the shelf life?
I have a bottle stored in the dark and in a controled temperature invironment and it is turning from yellow and starting to look brown. The bottle has never been opened.
I have a bottle also with a little left in it. This bottle
Was stored where not direct light could get to it but it was not in total darkness like the other.

[skinnykenney]

Bump.
Should I get in touch with the research place for a answer or does anyone have any idea what is happening to the letro?