The main question is does weather ball get boosted from 50 BP to 100BP in a rainbow field when there is weather?
The relevant effects of rainbow field on weather ball: Weather ball damage * 1.5, special normal moves * 1.5 and receive a random typing.
As we know, weather ball, in weather, changes from 50 base power to 100 and changes typing. Sans rainbow field, a Castform weather ball is 100 BP * STAB * Weather effect (e.g. 50% fire boost in sun), making it a potentially 225 power move.
With testing in rainbow field, I found that Weather ball's typing is no longer determined by weather; it seems to have been overridden by the field effect of a random typing (e.g. weather ball in sunny getting not very effective on Carvine and Pidgey).
Since weather ball is treated as a normal type move, does it even get boosted from 50 BP to 100BP in a rainbow field? The worst case scenario is weather ball does not get boosted to 100BP, and just gets the 50% field boost and 50% boost to normal special attacks, making it 112.5 power and probably no weather or STAB boost. In this case, weather ball is actually much worse in a rainbow field than out. If weather ball does get to 100BP, it is still at 225 power, but without predictable typing. Ideally, weather ball should be counted as a none-normal move when used in weather, so it just gets the 50% move specific boost on top of its 225 power (and lose the 50% boost to normal moves and random typing).