Largely a reconstruction. You can see the original stonework, but the pillars are reconstructions, as is the fragment of fresco. Sir Arthur Evans found evidence for the shape of the pillars and colours in the frescoes in some of the surviving frescoes and from illustrated tiles and sculptures of houses and parts of the palace (and from fragments of colour remaining on the walls).
Here's an interesting video showing what the Palace probably looked like originally, I recommend switching the sound off - it is only rather inappropriate music:
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