Colourfastness

It's got to be Faber Flags:

Faber Flags achieves the highest possible weatherfastness in the flag business:
6-7, on a scale of 1 to 8

It is vital with parasols and flags that the colour requested by the customer is not only printed in the correct shade, but also that the colour will be resistant for as long as possible to the effects of air, water and sun; in other words, it is important that the colour remains weatherfast for as long as possible.

Weatherfastness is expressed in values from the 'Blue wool scale', whereby a piece of wool is printed with 8 types of blue colour and rated for (weather) fastness on a sliding scale from 1 to 8.
1 is the worst value and 8 is the best.

Our inks achieve a value of 7 as standard, and a minimum of 6.  

Technically speaking, as a rule of thumb one point higher on the weatherfastness scale equates to a doubling of the usage time (= UV -/+ influence of moisture) with the same discolouration result. Say, for instance, that Faber Flags has a value of 7 and a third party has a value of 6. In this case it takes about twice as long for the colours used by Faber Flags to reach the same level of discoloration as the colouring agents used by the third party.

Download the full explanation here, along with the ISO standards.