Update colors.md

[AlexandraMasson]

Description

Update SwiftUI colors documentation with additional info on creating custom colors and using them throughout consistently.

Issue Solved

SwiftUI: Colors (Issue #4495)

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Type of Change

• Updating the documentation

Checklist

• [X ] All writings are my own.
• [ X] My entry follows the Codecademy Docs style guide.
• [X ] My changes generate no new warnings.
• [X ] I have performed a self-review of my own writing and code.
• [X ] I have checked my entry and corrected any misspellings.
• [X ] I have made corresponding changes to the documentation if needed.
• [X ] I have confirmed my changes are not being pushed from my forked main branch.
• [X ] I have confirmed that I'm pushing from a new branch named after the changes I'm making.
• [ X] I have linked any issues that are relevant to this PR in the Issues Solved section.

[CLAassistant]

All committers have signed the CLA.

[Radhika-okhade]

Thank you for contributing to Codecademy docs. The entry looks good to be merged.

[github-actions]

👋 @AlexandraMasson
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