Peter,
Agreed, you can't be too careful with fuel.
Both my bowser and storage tank are stainless and insulated and I have a Flightline Supplies big black filter in the delivery line. The filter drain is always blue avgas and I check the filter element a couple of times a year. It still looks OK.
Thats probably why my fuel stays blue.
Phil
EDIT
I have just had a delivery TODAY of fresh avgas into an empty bowser. Ill do the experiment of leaving two samples, one old and one new, open in a jar to see if it goes green. Ill post the results here in a day or two. Watch this space. (I might not be able to do the sun bit cos it constantly pisses down here
)