It is difficult to justify the comma, but anything else would look unwieldy and equally difficult to justify. The exclamation mark is also difficult to justify in purely grammatical terms.
Whilst I wouldn’t classify ‘Warning’ and ‘flammable’ as a true interjections, they are – in my view – closer to interjections than any other part of speech. ‘Warning’ is probably a true interjection now I think about it. That justifies the comma. You are seeking impact from your exclamation mark, though flammable is neither interjection nor exclamatory sentence (e.g. Run!). It is an exclamation in its own right though, so I agree with the exclamation mark.
In sum, I am happy with the punctuation. You could wimp out and use: