Y for yellow, given that's the color of the handle and that you need something to be representative of the color. Even having a yellow square could confuse it with S for square, or having a banana can confuse it with B for banana. Similar problems exist with S for Sun, C for (a yellow) crayon etc.<p>Also the handle is what has been drawn superimposed over the letter.<p>Still, confusion with A for axe isnt any better than S for sun or whatever. Maybe the reasoning was that axe was semantically far enough away from being one of the usual standard bearers of the color yellow, that children would associate the letter with the color rather than the object?