[PHP 7.4] curly brace usage with strings

Problem/Motivation

Checking for PHP version 7.4 compatibility yields this:

FILE: colorfield.module
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FOUND 0 ERRORS AND 12 WARNINGS AFFECTING 6 LINES
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
 137 | WARNING | [x] Curly brace syntax for accessing array elements and string offsets has been deprecated in PHP 7.4. Found: $raw_value{0}
     |         |     (PHPCompatibility.Syntax.RemovedCurlyBraceArrayAccess.Found)
...
(etc.)

Steps to reproduce

Proposed resolution

Replace curly braces with square brackets. Functionality should remain exactly the same.

Remaining tasks

User interface changes

None

API changes

None

Data model changes

None