Hello,
I bleach gray CDC with Hydrogen peroxyde (no ammonia so no fume) and it give good results .
Try to find 15% solution (the one used to bleach hairs) if not available the 3% would do but it would take much longer. Wear gloves and eyes protection. Use a glass container.
Wet the feathers with soapy water, rinse and add the feathers to the hydrogen peroxyde solution, stir . Depending of the %, it can take several hours, take samples, rinse and dry to assess the color. The feathers become light to dark tan color.
Concentrated hydrogen peroxyde can be difficult to get as it can be used to make explosives.