How do I dry moringa leaves properly to make good quality powder?

Colour loss usually means the leaves were dried in direct sun or at too high heat, or were dirty or wet when dried. Good moringa powder needs shade drying or low temperature drying so the green colour and nutrients are preserved, clean leaves washed and well drained before drying, and proper hygiene to avoid microbial contamination which is a major rejection reason for export. Dry until the leaf is crisp, then grind and sieve to a fine clean powder, and pack in moisture proof packing because powder picks up moisture and spoils. Direct sun drying is cheaper but gives the yellow brown low value powder you describe. If you are targeting the export or premium market, invest in proper drying. Ask your KVK or a food processing centre about suitable low cost dryers.