Note sure if you can get the Dreamweaver extension to work in LiveCycle Designer.
Attached is an example where clicking a button assigns that colour to a global variable and to the captions of the fields. You can have the script in mouseEnter if you want. The second option is allowing the user to select the RGB and then this is automatically applied to selected objects.
A lot depends on how many colours you want and what elements you want in the form to take up this colour.
ps just updated a version with "sliders", which set the colour on the change event.