useColors: fix contrast check

[ellatrix]

Description

Introduced in #19046.
Props to @epiqueras for finding the problem in #19474 (comment).

Instead of checking the node type, the node itself is checked.

How has this been tested?

Create a paragraph and make the text colour white (with a light inherited background). Type something in the block to trigger a change (it's strange that this is necessary). The contrast warning should be displayed.

Screenshots

Types of changes

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR. .