Why the blue numbers/letters wears off so quickly?

[watchcollector]

I had two watches in the past with white dial and blue painted numbers (the IWC F.A. Jones and the Sinn 6100 Regulateur), and both of them had this problem - the blue painted numbers/letters started to wear off quickly after I bought them. Can you explain me why? Does the blue colour wears off more quickly than the other colours? Or it's just about the model of the watch, some models are crafted more poorer than others, for example... Or which should be the cause? Did this thing ever happened to you? Which of your watches had this problem? Is it the exposure in strong natural light, the differences in temperature, or what else can cause this?

[frankt]

Blue pigmented dyes are traditionally the least resistant to fading from exposure to direct sun light.......They are also very sensitive to atmospheric ozone fading, but in a sealed watch environment that shouldn't be a factor....

[watchcollector]

So you say that maybe those watches were not sealed too well, or I simply had bad luck...or what?