Boiled cheese ravioli for 10 minutes. Added sliced yellow squash for the last 3 minutes. Nothing else in the pot but boiling water, not even salt. It looked fine cooking but after I drained everything in the colander, the squash rind stayed yellow but the flesh turned bluish-green on most pieces. Not all pieces though. It tastes the same as usual. The color is freaky. Why did this happen?