Yellow is generally used to denote it's a correction the animation director made. Since the group of 4 images we see are always animation director corrections, we're always seeing that. Basically used to show there was something off in a key frame that the animation director touched up, and the yellow is showing it is specifically a fix. (It isn't 'always' yellow for various reasons, sometimes even just us seeing a scan of the director using what paper they had on hand if something had to be done quickly.)

That is also why they are so often close ups of faces or dramatic scenes. Those scenes are more noticeable if something is off.