As you can see, there is clear boundary where my repair work meets the non-repaired area of the board. Is this caused by sanding? Or is it caused by slightly different resin colour? How does one avoid such an ugly look?
(full disclosure I played around with the contrast and brightness on the photo to make the issue clearer - also thats why the board looks kinda ghostlike).
I can’t see anything of note in your picture. If your board is suntanned to begin with removing the amber resin is going to yield a color change. It is what it is. Just surf it or add some art.
There is a clear line that runs across the board about 6-8 inches towards the nose from the fins. What is weird is that the board is pretty white, and the new repair resin is darker!
Get ready to go down the rabbit hole of color matching! 
What resin are you using? Silmar 249 is what most boards are made with. It cures water clear with a very super slight blue or green to it depending on optical brighteners.
I’m using Shapers UV cure poly. It does have a slight blue tint. Perhaps that is the cause.
Ding repairs are difficult to match invisibly. Do the best you can, if it turns out invisible be proud of your repair, if it shows up plain as day (the usual case) let your board wear it with pride like an old war wound.
