Update colors.css for new perl logo color

[thibaultduponchelle]

Related to https://github.com/exercism/website-icons/pull/192 and https://forum.exercism.org/t/new-logo-for-perl5-track/17232/2

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[thibaultduponchelle]

@iHiD Thank you for reopening, this PR should be ready to merge 😃

For your information, there's an alternative to this PR at https://github.com/exercism/website-icons/pull/193 where I reinstall old color (along with other good stuff: cleanup hidden SVG objects and optimizing SVG). Feel free to pick one or the other.