How do I add a color hex code field to a product attribute using drupal commerce 2?
For anyone else grappling with this issue in future here is what I did:

****How to add a color hex code field to a product attribute using drupal commerce 2****

see also: https://drupalcommerce.org/blog/44310/enabling-fancy-attributes-commerce-2x

Use the Color Field module to add a color field to your Color attribute. 
https://www.drupal.org/project/color_field

Install with Composer: $ composer require 'drupal/color_field:2.x-dev'

+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

Product Attributes  >>  Add Colour Attribute    >>  Manage Fields

Add: 

Label:      Color Hex Code  

Machine Name:   field_color_hex_code    

Field type:     color

DEFAULT VALUE
The default value for this field, used when creating new content.

COLOR HEX CODE

Color       #D3D3D3

Opacity     1

Record Opacity  Ticked

FIELD SETTINGS TAB

Format storage  #123ABC
Choose how to store the color.

Allowed number of values:   Limited     1

+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

Product Attributes  >>  Add Colour Attribute    >>  Manage form display

FIELD   WIDGET

Name        Textfield       textfield size: 60

Color Hex Code  Color default       No placeholder

+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

Product Attributes  >>  Add Colour Attribute    >>  Manage display

FIELD           LABEL       FORMAT

Color Hex Code      Hidden      Color Swatch        Square Width: 40 Height: 40

Note:
Remember to change the attribute selection widget to use the rendered option for your attribute.