Changed color utility functions to maintain color format supplied

[cchaos]

I noticed in our Global Values page any time we were doing color calculations on hex values, the rgb() value was output which in turn confused the color picker. So I've updated these functions to attempt to maintain the same format that was passed in except for makeHighContrast() since we don't support alpha values anyway.

Before

After

Color utility page showing input vs output

Checklist

• Check against all themes for compatibility in both light and dark modes
• [ ] Checked in mobile
• Checked in Chrome, Safari, Edge, and Firefox
• [ ] Props have proper autodocs and playground toggles
• Added documentation
• [ ] Checked Code Sandbox works for any docs examples Emotion is not working
• [ ] Added or updated jest tests
• [ ] Checked for breaking changes and labeled appropriately
• [ ] Checked for accessibility including keyboard-only and screenreader modes
• A changelog entry exists and is marked appropriately

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_5230/

[thompsongl]

LGTM
I pushed a commit with a little helper function

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_5230/