That is normal. It seems like many folks have this concern. What you are seeing is the reflection of red wavelength (~620-~740nm) light from erythrocytes coursing through the microvasculature that feeds the new tissue at the growth margin. It recedes as time passes and the tissue matures further. It easier to observe in tortoises with lighter-colored growth.
Be more concerned if there is a truly bright pink or red line, which may indicate a septic issue. In such a case, it will likely appear "angry" and there is often hemolysis in the area of infection.