The color of a rare is commonly yellow/gold. To find this out, take a card and look at the company symbol. If the color on it is yellow or orange, its a rare. It is sometimes hard to tell the difference between the orange mythic rare and the golden rare.
Alpha and Beta Sets
Back when there was still Beta and Alpha, there was no mythic rare cards; they weren't made until later. For example, Black Lotus and the Moxes are rare because it is from these sets. All of the Power Nine are rares, though by today's standards they would be mythic rares.
Rare cards are well known as being stronger than most cards, with the exception of Mythic Rares. Their abilities often are more game altering than most other cards, with the exception of Mythic Rares or Planeswalkers.