[4.0] field simple color fix

[dgrammatiko]

Pull Request for Issue # .

Summary of Changes

• Revert the removal of the rgb to hex function

Testing Instructions

Apply patch and check that the selected value is retained after save (template autum has a field for testing)

Expected result

Actual result

Documentation Changes Required

[anuragteapot]

I have tested this item ✅ successfully on 53a90dc

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[tassosm]

I have tested this item ✅ successfully on 53a90dc

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_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/20146.}