How to grey out column in NewForm.aspx

Question

Good morning,
I have created a custom list.  In this list, I have a Drop-Down Column with various options and also a Text Column. 
What I am hoping to achieve is that if, for example, if Choice 3 is selected from the drop-down column, then the Text Column becomes "available".  Otherwise this column remains greyed out.
I am no master at coding so hopefully a solution that is not too complex.
Thanks!

Answer 1

If you're building out the form in InfoPath, you can also do this with some simple rules.
Load the form in InfoPath, and select the textbox control
Select 'Manage Rules' from the Home ribbon
Create a formatting rule for the text box as follows
Conditions = If dropdown column=blank, OR dropdown column=option 1, OR dropdown column=option 2 (see below screenshot)
Select the 'Disable this control' checkbox
I also recommend changing the color of the textbox so that it is greyed out, otherwise people could be confused as to why they can't edit the text field. You can do this in the same rule, with the paint bucket icon
(Where 'session' is your dropdown column and "Option 1" and "Option 2" is the text for your first two options.)
If you want to provide even more user guidance, you could add some text underneath the textbox which is only displayed when the textbox is disabled, saying something like "This textbox is only available when option 3 is selected". To do this, add the text
to the form and then add a formatting rule to the displayed text:
Conditions = If dropdown column=option 3
Select 'Hide this control'
Hope that helps!