The yellow ones are over-ripe.
Budding G wrote (see)
Just read about yellow cucumbers and it could be cross pollination I've got yellow as well and it has been said that courgettes and squash being too close can cause this and make the cucs yellow and bitter tasting...
The cross-pollination doesn't affect the flavour of this year's fruit - it only affects the fruit grown from seed which is the result of cross-pollination. If you don't grow plants from your own saved seed there's no problem with growing courgettes and cucumbers near to each other.