Phal leaves firm but slowly yellowing

[Catriona]

Hello, I'd like some advice please. This orchid has been blooming since beginning of October when I bought it. I've recently noticed that the leaves are yellowing; the 2 bottom ones v. much so (they're also quite soft), and the others some from tip, some from base, one across the middle. I don't think the roots look as good as they could do - perhaps I've been overwatering? - although no more than the others. Any suggestions much appreciated!