Thinking about graying out buttons

A few days ago, a colleague was working on a form filling page and mentioned whether the "Finish" button should be grayed out. My first reaction was that it was not recommended to gray it out, because I thought of several scenarios I had encountered:

1. There are many items in the form, and I will select the required items before submitting. However, the button does not light up when I select the required items, but lights up when I do not select them.

The button not only has the function of submitting, but also serves as a verification and prompt. When it is grayed out, users need to guess which item needs to be filled out.

2. The login button of many apps is grayed out. When you start to enter your password, the button lights up. At this time, you really want to log in immediately, but the password is not completed.

The effect of the button lighting up can gradually guide the user to perform the operation, but the number of digits in each person's password is different, so when the button lights up is a problem. Lighting up too early will disturb the user.

3. iOS9 Music account information, I entered the same information twice, but the "Done" button status is different. (Just want to complain)

The button is grayed out and I don't know what's wrong with the format.

Should buttons not be grayed out? Not really. Here are some good scenarios for graying out buttons:

1. Step-by-step registration by mobile phone number

2. Payment password

There are also many apps that have omitted the "OK" button, which will not be discussed here.

3. The interface of the express delivery machine (same as above, the format of the pickup password is a fixed 8-digit number)

In the above three scenarios, there is only one input box + button, the input content format is fixed, and the user knows clearly how to input.

Doing so not only guides users to perform operations step by step, but also eliminates the error message when the form is submitted empty. This is the truly meaningful graying out.