In the pink means healthy, well, or in excellent condition. People usually use it for health, but it can also describe something that is working very well.
The expression probably does not come from the color of human skin. A more likely source is pink, the flower. Its neat, scalloped edges were described as pinked, and the flower became linked with the idea of something fine or perfect. That helps explain an older phrase, in the very pink of condition, which meant being in top form. Over time, English shortened that to in the pink.
Today, the meaning is simple: someone or something is in very good shape.
- After her surgery, she is back in the pink.
- Even after 20 years, that bicycle is still in the pink.
This is a slightly old fashioned expression, but it is still easy to understand. If you use it for a person, it suggests strong health. If you use it for an object, it suggests excellent working order.