According to Winnie the Pooh and the Honey Tree, this was possible because Eeyore is full of sawdust. Also, when Eeyore appears in animation he is colored his natural grey, though he is coloured blue with a pink muzzle in merchandising.
Tips for printing coloring book page or coloring sheet:
Step 1: Click on "Print" link of your choice, if you want a
Eeyore pictures image for coloring yourself then you need to click on "Print
Eeyore pictures Coloring page (B/W)" link. If you want colored (already filled with colors)
Eeyore pictures graphic to print then click "Print
Eeyore pictures Coloring page (Color)".
Step 2: Now a new pop-up window will open along the
Eeyore pictures image of your choice (if new window does not open-up then you may need to check your pop-up blocker settings.). Now click on "PRINT". This will open printer dialog, select your print and print.