From Leopard Gecko Wiki
The Red Stripe trait is a line bred trait which results in a gecko with an orange or yellow body with two red colored stripes outlining a dorsal stripe. The Red Stripe morph can vary from geckos with intense outlines to some with less of an outline.
When hatching Red Stripes, they tend to have a brown color where the stripes will be coming in. It takes approximately 6 months for the stripes to fully come in.