fix: update color property accessors to match Flutter's stable API

[deandreamatias]

💡 Motivation and Context

To avoid a info lint about deprecation.
Related PR on Flutter.
This would improve the pub points on pub.dev

#skip-changelog

💚 How did you test it?

When applied the change, the lint was solved

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed.
• No breaking change or entry added to the changelog.

[marandaneto]

thanks @deandreamatias