When your computer first starts up, it initializes to a known good state. A lot of problems that happen in software are a result of things ending up in a bad state. By turning it off and on, you return to that original good state.

Take a game of chess for example, when you start, all the pieces are in known positions and you can start playing the game (good state). Now, say halfway through a game, someone bumps the table, and scatters all the pieces around (bad state). You don't know exactly where everything was, so the only fair thing to do is clear the board (turn off), and start a whole new game (turn on), thus returning the pieces to their starting positions (good state).

So 3 reasons:

1. Gamma correction - old school tube TVs work differently than modern LCD/LED screens. Specifically in the way the pixels represent color intensity. With modern screens, there is a mostly linear relationship between the pixel value and the actual color being seen so red 200 is twice as red as red 100. With tube TVs there was a curve, so red 200 might be 3 times as red as red 100, so you'd need to adjust the value down so that it looks right on the screen.

2. A lot of animation was done by hand - a lot of older animation was drawn on physical media, photographed, then assembled into a video reel. This means that the physical color palette used was restricted to what the artists had available at the time. The process used today is almost entirely digital and software can create basically any color you could ever want.

3. Style - in a lot of cases, the darker coloring was a stylistic choice. For example, scooby doo has very dark coloring to add to the mystery/horror atmosphere presented by the show. This is still done in some modern cartoons & animated shows.