That hair blend is yummy!
Personally, I like Dash's colour with it in *both* photos (allowing for different monitors, with/without flash, different lighting etc.) I plonked my Daddy Celebrations on top of each of my G3 Dash's heads and it looks pretty!
It depends though... if you are after more contrast then I'd agree that a white base would be better. But as it is? I think it looks really nice, and is definitely a good candidate for some kind of water inspired theme
Also, are you going for a literal or more abstract inspiration from a koi?
If you're going to be adding actual Koi to the pony (sculpt or symbol), and give her a general Japanese feel, then who says the koi has to be orange? Look up "koi fish" on Google or any image search engine, and you'll find loads of nice pictures (the 2nd one, if you use Google, is especially nice!). You could make the Koi golden, orange, black, white, spotted, blue... you could make the darn thing pink or purple or rainbow if you wanted! The koi doesn't need to be a realistic colour at all - they have such a distinctive shape, such evocative imagery (I know I think of gorgeous Japanese buildings, gardens and blossoms whenever I think of Koi!) that you don't need to stick with a conventional colour at all.
If the pony herself is themed around a Koi... again, she doesn't have to be either a merpony or orange or white or whatever. I'm into more abstract concepts in that I don't either copy character costumes to the letter, or take the theme/idea of "fire" and just paint flames everywhere.
Think of the colours or imagery a Koi gives you, the mood that stuff puts you in etc. Write it down if you have a memory/attention span like mine
She could have gradient or scaled legs or face in gold or white, her hair could be pinned up with two strands hanging down like the koi's funny little moustache thing, she could have koi in her hair... soooooooo many possibilities! If you want to stick with Dash, just add "blue" in front of your koi google search!
I also find that sketching out colours and ideas really
helps. You don't have to print out a base - just a scribble will do. Personally, I find that printing off pony bases from deviantArt can be quite restrictive as they have set shapes and poses and eyes and hairstyles and moods etc.
It doesn't have to be a work of art either - nobody will see it! I can tell you that none of my sketches are more than about 2", and hardly any of them really look like ponies - they're all weird deformed creatures with long necks and short legs XD But you can play with poses and get an idea of the mood/personality of the custom you're working on. I find that actually helps a lot too - even though my sketches are horribly deformed, if I have a bouncy pose for a candy themed pony I can get more ideas down quickly because she's just... she's setting the mood for me!
All you're doing is seeing if the colours match, getting an idea for a symbol (if there is one), getting the hairstyle you want etc. My sketches of curly/wavy hair ponies are just scabby little scribbles with childish swirly bits for hair
Often you'll get a final idea, and when you're finishing up the pony you'll think of little other things along the way anyway
Sorry for the essay LOL this is just how I work XD