Things that are obvious once pointed out, but I never thought of on my own.

The colors of the rainbow. How many are there?

Red, orange, yellow, green, blue, indigo, violet. Seven colors.

Do you see anything unusual about that list?

How many primary pigment colors are there?

Red, yellow, blue. Three.

How many secondary pigment colors are there?

Orange, green, purple. Three.

For a total of six.

How come we say "purple" for a mixture of blue and red, but split "purple" into "indigo" and "violet" for purposes of the rainbow?

Because Isaac Newton decided that there MUST be seven colors, because there are seven classical planets, seven notes in the classical scale, seven days of the week, and seven is just generally an important number. Therefore, there MUST be seven colors, and therefore, "indigo" and "violet" must be separate colors, and not just shades of "purple".

Because, if the facts don't fit your theories, the facts must be wrong.