Steps to reproduce the issue

• Open a templateDetails.xml for any template (Cassiopeia)
• Paste inside <fieldset name="basic"> the following sample field

<field 
      name="color_fill"
      type="Color"
      label="Color onchange test"
      format="rgb"
      default="rgb(0, 102, 153)"
      onchange="console.log(this.value)" />

Expected result

The browser console throws logs of rgba() values.

Actual result

Nothing happens as the markup of the field is wrong, see below (some attributes removed)

<input
  type="text" name="color_fill" id="color_fill" 
  value="rgb(0, 102, 153)"
  placeholder="rgb(0, 0, 0)"
  class="form-control minicolors rgb minicolors-input"
  data-position="default"
  data-control="advanced"
  console.log(this.value)=""
  data-format="rgb" data-opacity="1.00">

System information (as much as possible)

Joomla 4.0.2, I think this issue is also present in Joomla 3.x as well.

Additional comments

I might commit some possible fix, anyone interested in testing and pushing this into the stable release?