Acorn Squash - When to pick?

I am new to gardening and new to blogs. But here goes. I planted acorn squash and I have a good crop. I have been picking and enjoying the dark green ones. My problem is that I have lots of very large acorn squash that look mature but they remain a bright yellow. Are they ripe or will they eventually turn green?
Thanks for any help. :)

a good hint with any winter squash is, either after the first frost, OR, harvest when the stem connecting the squash to the vine starts to dry and brown. The plant is telling you it is ready.