Are all peaches yellow inside?

The answer

No. Peaches can have yellow or white flesh, and the two types tend to differ in flavor: yellow-fleshed peaches usually taste classic tangy-sweet, while white-fleshed ones are often described as milder and more floral. The red blush on the skin is not the best ripeness clue; check the ground color beneath it, since a green undertone usually means the fruit was picked too early. Nectarines have the same two flesh colors.