André decided to play "gotcha" by noting that not all of the notification icons went colorless and wondered what the criteria was for deciding which ones to make colorless and which ones to make colorful.

It's very simple: The design team generated colorless versions of the most commonly-seen notification icons. They didn't have the time to generate colorless versions of all notification icons, so they focused on the ones that gave them the most benefit.

This is the standard tradeoff you have to make whenever you have finite resources. Eventually the marginal benefit of redrawing one more icon exceeds its marginal cost, at which point you stop. The marginal cost is measured not only in actual resources (designers can redraw only so many icons per day, and you have money to hire only so many designers) but also in opportunity cost (time spent redrawing icons to be colorless is time not spent on other design tasks).

This is the same reason that not all icons in Windows XP were given the full-color perspective-view treatment. For example, nearly all of the icons in the Administrative Tools section are the old Windows 2000-style 16-color flat (or isometric) icons. The design team focused on the 100ish most commonly used icons and went to the effort of redrawing them in the Windows XP style. The more rarely-used icons were left in the old style because the cost of converting them did not merit the benefit.

The same thing happened in Windows Vista, when the icon design changed yet again. The style became less stylized and more realistic, but not quite photorealistic, and the angle of presentation changed. The design team had the resources to convert the most commonly used icons, and the rest were left as they were.

It's the Pareto Principle again. If you have finite resources (and who doesn't) you may find that you can get 80% of the benefit by doing only 20% of the work. And that leaves 80% of your capacity available to address some other problem.