I have sown a few different kind of peppers. Some germinated 100% after 4 days. Some did not at all. So it may be the type of pepper, or the seed quality.

My birdseye pepper seedlings were tiny little and the others were 'normal' size. So this may also be attributed to the type of pepper.

The purple coloration is from anthocyanins produced as a type of sunscreen to protect the seedling from intense (sun)light. If the seedling appears healthy it should be no problem.