Chandra’s second ability checks the characteristics of the card you exiled to see if it’s red. If the card is red, but the spell that card becomes somehow isn’t red, you may still cast it. For example, Rowan, Scholar of Sparks, the front face of a modal double-faced card, is red. Will, Scholar of Frost, the back face of the same card, is blue. In exile, that card is red, so you may cast either face if you exile it with this ability.