fix: improve regex to check if string is hex or rgb

[Thunderbottom]

use a different regex to check if passed string is a valid color by checking for both hex and rgb/hsl values. also, use a case-insensitive regex for checking hex colors.

fixes #274

[coveralls]

Pull Request Test Coverage Report for Build 155

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 38.158%

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Totals
Change from base Build 153:	0.0%
Covered Lines:	29
Relevant Lines:	62

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💛 - Coveralls