I made a large contribution to a Colorado 529 plan that was sent 12/30/17 but processed 01/03/18. Does the state of Colorado consider this to have occurred in the 2017 or 2018 tax year?

2017 tax year.

I am using TT 2017 desktop and for the life of me I can't find where to add Colorado 529 contributions. Can anyone help?

You will see it on the page 'Here's the income that Colorado handles differently'. Title Education.
Let me know if you find it.

I am working with desktop version (not the web version) - but I just cannot find where to add the Colorado 529 contributions.  I cannot get to this page mentioned above "Here's the income that Colorado handles differently"

Click <STATE TAX> tab ==> <Continue> ==> on "State Returns page", click <Edit> ==> You now in this page "Here's the income that Colorado handles differently", and at 3rd item is
 "Education" - 529 College Tuition Program Contributions. This item "Education"will disappear if you have any value enter, and it will display as the top item "529 College Tuition Program Contributions" with the contribution dollars. Hope that help.

Sorry, but this flat-out not an option in the desktop version of TT 2017.  There are lots of things listed, but 'Education - 529 College Tuition Program Contributions' is NOT one of them.

I found that you can get back to the "Here's the income that Colorado handles differently" page if you click through to the end and find the button that says "Go over Colorado again"and keep clicking "Continue" until you get to the page.

A related question: Does the Colorado 529 contribution go in the box on the page "Colorado Subtractions from Federal AGI" ??  I contributed about $5,000 to the 529 and the Federal amount reads "$5,000". The Colorado box reads zero at the moment.

I t was not on the Colorado section! It must have been added late.  It was necessary to up Date Colorado to get it. You or at least I could not up date Colorado via Wi-Fi . I had to add a either net cord so I was not on wifi then instantly it up loaded.  The internet is running to slow, due to CV19 and everyone is home on the internet.