|my cut leaf-philodendron, is turning yellow once the leaves open|
|Yellowing on a philodendron can be caused by different factors so I will mention some of the most common. Sometimes the new growth is a slightly different color until it matures a bit, but it should not be literally yellow.
This is a plant that does well without direct sun. Direct sun can cause it to "sun burn" and yellow.
Yellowing can be due to overwatering, this would be most noticeable at the base of the plant first. It can also be due to underwatering, but if this is the cause it should also be wilting. Philodendron should be allowed to dry slightly between waterings. The pot should have drainage holes in the bottom, and any excess water should not be allowed to accumulate in the saucer under the plant -- empty the saucer after watering.
A slight yellowing or pale look can be caused by lack of fertilizing, but yellowing and browning foliage can indicate root damage due to overfertilizing. You would use a water soluble fertilizer for foliage plants such as 10-10-10 plus minors from spring through fall per the label instructions. When growth slows for the winter, stop fertilizing, then resume in the spring.
Yellowing can also be caused by insect damage such as spider mites; check carefully for signs of fine webbing on the undersides of the foliage. If you find signs of that, try spraying with commercially formulated insecticidal soap per the label instructions.
I hope this helps you trouble shoot.