Actually, I tried to say that, if you were to average over all distributions -- all equally
likely in the absence of information -- then you get the uniform distribution. So, in that
sense, I didn't just pick it out of the air. After all, that bag may have been filled by
someone who has a marked preference for pastel colors and added some white ones for balance.
Or, perhaps, it is a bag made by someone who was filling bags with balls of colors in
alphabetical order and he started this bag with Ultramarine (U) ,Violet (V), White (W)
but, since he couldn't come up with a color starting with X, he got frustrated and just
stuck another White (W) just to get four in there! I give each distribution (whether binomial
or even one conveying a very personal bias) equal weight.