The fluff or lint that you get in your belly button, Always blue in colour regardless of t-shirt/shirt colour.
There are many theories on this topic, One being that the colour of the lint is in correlation with the colour of garment you are wearing at the time. This is NOT true, if you see photographs of different coloured lint, they have been photoshopped or Paint shop pro'd (depending on which you prefer, Lesser minded people usually prefer Paint shop pro studies have been done).
person 1) OMG I have pink naval lint in my belly button. . .
person 2) Dude no you don't, it has to be blue.
Person 1) Yep it does, I photoshoped this, harhar har. . ..