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Pressure grows on Apple to open up iMessage
(finance.yahoo.com)
This is a most excellent place for technology news and articles.
Two related issues are being confused/conflated here.
The first is the American cultural significance of the green and blue bubbles. This is the thing that Europeans generally don't care about as most are using WhatsApp et. al.
The second is the lack of interoperability between chat protocols such that it degrades the experience for everyone. This is what the EU is targeting.
I don't think the colours of chat bubbles for specific devices as displayed by other specific devices falls under that remit. The implementor must comply with providing the same service level though. Whether or not this will lead to less cultural significance for bubble hues in the US remains to be seen.