system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does yellow food coloring look red when it's concentrated?
gpt
A dude named Beer made a law about it. It's called "Beer's Law", oddly enough.

Here's the way it works. Say you have a rainbow and a very special pair of filtering glasses.  Without the glasses, you see every colour in that rainbow. But the glasses make the lower part of the rainbow a little bit hidden more than the upper layers. So with them on, you see lots of the upper part (which is reddish) but not quite all of the lower part (which is bluish)... and yellow's in the middle so it's a tiny tiny bit missing but most of it's there.

Now, put on another pair of those glasses so you look quite dorky with two pairs of glasses on. Now almost all of the blue is gone.. and quite a bit of the yellow's gone too.

Now put on a third pair of glasses so you look super dorky. Blue's pretty much not there, yellow's about all gone... and so you largely see red.

The more layers of dorkiness-causing glasses, the more red shows through and the less yellow does. Well, Beer's Law is kinda like those glasses when it come to the liquid in food colouring. The thicker it is, the more it absorbs yellow light and only shows red light, even though the base colour of the food colouring is yellow when diluted. 

In very simple terms, the more layers caused by more concentrated liquid that the light passes through, more and more of the reflected light off of it that you see shifts toward red because all of the blue and almost all of the yellow gets absorbed. 

But water that food colouring right down or mix it in icing so there's just a tiny tiny layer of light-reflecting stuff at the front, and the yellow now shines through because it's not really absorbed at all.