I agree with Ice-cold's choices here, in that 2, 4, and 6 are the best options. Despite Ice's resistance to commenting on 4, it does look pretty good.well this is a big post
To compare yellow-skins in general I prefer #2 over #4.
On the subject of drastic color changes, I really like #6, but after browsing through the BW Pokedex I found it impossible to find any shiny-evo-chains that had that much variation. The general theme with shiny pokemon in Gen 5 seems to be, keeping one shiny palette the same and changing the other one. (Pretty often they even use the exact same color palette for both parts) So while its less 'fun', 'creative', and 'original' to use the same shiny colors, it does match the standards of Gen 5.
Summary/Conclusion: My best idea is to use Tomohawk's shiny skin color and either Black or Purple feathers, whichever you prefer.
EDIT: Or if you really want to be creative you can keep Tomohawk's feather color and find a skin color that doesn't make someone want to tear out their eyes.
EXAMPLE OF ~SAME COLOR: Solosis/Duosis/Reuninclus
EXAMPLE OF SINGLE PALETTE SWAP: Ducklet/Swanna