Drybrushing is the key!
Start off by deciding on a colour scheme you like and think about which parts of the wing you want to paint which colours. The new Hobbit Eagles for example have three rows of feathers with different lengths, maybe you want to use a few different colours, for example brown on the top row and black on the bottom. Once you have decided on a colour scheme, paint the area with a coat of the colour you want, for example a dark brown such as Bestial Brown/Rhinox Hide. Once that has dried, drybrush the area with a lighter brown shade (for example Graveyard Earth/Steel Legion Drab. Make sure you don't have too much paint on your brush! Finally, take an even lighter shade of brown (Kommando Khaki/Karak Stone, for example) and lightly drybrush the area a third time.
That's basically what I did on the Eagle in my picture, if you have any other questions, hit me up
