TLDR; They were probably not horribly painted given the evidence from other types of surviving artwork from the period

It's a shame to TLDR the author's own excellent summary:

There is a single explanation for the fact that the reconstructions do not resemble the statues depicted in ancient artworks, the fact that their use of color is un­like that in ancient mosaics and frescoes, and the fact that modern viewers find them ugly. It is that the reconstructions are painted very badly. There is no reason to posit that ancient Europeans had tastes radically unlike ours to explain our dislike of the reconstructions. The Greeks and Romans would have disliked them too, because the reconstructed polychromy is no good.

And:

One wonders if something else is going on here. The enormous public interest generated by garish reconstructions is surely because of and not in spite of their ugliness. It is hard to believe that this is entirely accidental. One possibility is that the reconstructors are engaged in a kind of trolling. In this interpretation, they know perfectly well that ancient sculptures did not look like the reconstructions, and probably included the subtle variation of color tones that ancient paintings did. But they fail to correct the belief that people naturally form given what is placed before them: that the proffered reconstruction of ancient sculpture is roughly what ancient sculpture actually looked like.

Ah so it was an Ecce Homo situation?