First question: How flexible is the client?
2nd question: Do any of these colors have to be exact Pantone matches?
In a friendly way, I'd be trying to adjust client expectations toward what makes this a simpler print. And size of order + how much the client is willing to spend affects how simple I'd try to make it.
As mimosa said:
This is one of those situations where I would adjust the art to print better with fewer colors vs adding colors to represent "poor" art more accurately.
Mimosa's other advice is probably the tack I'd take, integrating specific Pantone colors if needed. Or, if the client isn't very picky, I'd consider doing 4cp just to make separating easy. (Is that lazy?
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...a n d you just posted while I was typing. Well, for what's worth, there's a little more input! Maybe 4cp would help with clouds. But one thing I definitely will add: be honest with the customer about what you can & can't do - controlling expectations goes a long way in keeping people happy.