I would basically try to match the hatch :)
Inshore halibut, the kind you stand a chance of catching on a fly rod, are feeding mostly on smelt and grunion, less on anchovies and sardines, and some on miscellaneous forage like small perch.

So to imitate the smelt/grunion a white sparkle belly with green or light grey back is good.

For the miscellaneous forage you might try a mix of olive and sparkle belly and a brown back or something like that.

Just go to goleta beach and look in the water on a clear day. You'll see the smelt on the surface in small schools, and the little perch sitting on top of the grass beds. That's what you want to imitate.