docs: fix broken imports in color utilities

[tkajtoch]

Summary

This replaces the auto-suggested imports from @elastic/eui/src with the correct @elastic/eui ones and fixes the new docs site builds.

QA

• [x] Confirm the "Build and deploy new documentation website" is passing
• [x] Confirm the /new-docs/docs/theming/colors/utilities/ docs are rendering as expected

[kibanamachine]

Preview staging links for this PR:

• Docs site: https://eui.elastic.co/pr_8106/
• Storybook: https://eui.elastic.co/pr_8106/storybook
• New docs: https://eui.elastic.co/pr_8106/new-docs

[elasticmachine]

:green_heart: Build Succeeded

• Buildkite Build
• Commit: dfa0c10035565eac1c886d67d1488a63f86e4919

cc @tkajtoch