Oncidium Twinkle still yellowing...

[greenpassion]

I had posted earlier in the month about my newly aquired Twinkle with yellowing leaves. The pb is also yellow. It is still in bloom, tons of blooms, but the leaves and one of the pbs is very yellow. I got a heat mat (just came yesterday) as someone said oncidiums need warmth at the roots. It's in the same plastic pot that it arrived in, with narrow slits in the sides, in a bark mix. I keep the temps around 62, and the humidity around 55%and now, as I said, it's on a heat mat as well. What should I do to stop this yellowing?? Do they yellow from too much water, or not enough? roll: