Someone had this as a card. I can't recall where though.

IMO that's a better cost for it.

Black gets to destroy creatures, red gets to destroy artifacts; so there's fair reasoning for that.

Both green and white get to destroy artifacts (white much less so, nowadays) - but neither get creature destruction (though white gets creature exile, which is better). So - why does this have the 'creature' limitation? And why white?

I could just about see 'exile target artifact creature' here; but that's still a bit too blunt for white.

I'd consolidate these into one card if there was a 4 color hybrid symbol. Also "destroy (any) target artifact" isn't something any color should be doing at 1 mana.

Well, indeed. This should probably be CMC2 at instant; if it's going to keep these colours. At 1 it should probably be a sorcery.

Instant speed CMC 1 artifact destruction does exist - Crumble. That has a downside on it, mind you. And was printed back in the days when removal wasn't utterly shite.

On the subject of 4-colour mana symbols - how about an inverse-video symbol? Would that look sufficiently "Any mana as long as it isn't blue"?

Though whether it's worth making; for the few effects that can be done in four colours but not all five.

There is , which is a 1 mana instant those does this and also has 2 other modes. Considering that this is worse than a 25 cent card in 2 ways, and only has the advantage of having a slightly less restrictive mana cost, I don't think lowering its power level would be necessary.