New growth simply hasn't colored up yet

What's happening

Croton leaves often emerge green or pale and develop their full red, orange, and yellow coloring as they mature in good light. A plant sitting in a bright window with vivid older leaves but greener new ones is almost certainly fine — those young leaves just haven't finished coloring up. True fading would show on the established leaves too, not only the newest growth.

How to fix it

Keep doing what you're doing and give it time. Maintain the bright light, keep watering even, and feed lightly through spring and summer to support healthy new growth. The young leaves should deepen into their colors over the coming weeks. If your older leaves are also losing color despite the bright spot, look instead at whether something has changed recently — a move, a draft, or a watering lapse — that's stressing the plant.

This is general guidance based on common symptoms; individual plants vary.