[TextField] Helper text should be in error color if both disabled and error are set to true

[jiayuandeng]

• This is not a v0.x issue.
• I have searched the issues of this repository and believe that this is not a duplicate.

Expected Behavior

If a TextField has disabled="true" and error="true", the helper text should be in error color.

According to Material Design Spec:

Error message
When text input isn’t accepted, an error message can display instructions on how to fix it. Error messages are displayed below the input line, replacing helper text until fixed.

Since error message "replaces" the helper text, it should always be in error color whether the TextField is disabled or not.

Current Behavior

If a TextField has disabled="true" and error="true", the helper text is in disabled color (gray).

Steps to Reproduce

Link:

1. https://codesandbox.io/s/ryw2w9m93n

Context

Your Environment

Tech	Version
Material-UI	latest
React	latest

[matthewdordal]

I can open a PR to fix this issue.

[samuelpucat]

What about border color of TextField variant="outlined" when both error="true" and disabled="true" ? Both variant="standard" and variant="filled" have border bottom red, while outlined have the same border as when only disabled. I can't seem to find it in Material Design spec. Maybe disabled and error state don't even go together.

Here is a comparison: https://codesandbox.io/s/material-demo-forked-ygq3um?file=/demo.js