How do I print an image field value in views-view.html.twig?
views-view.html.twig is not meant to be used for field printing.

You should use views-view-fields--[view-name].html.twig 

OR 

views-view-fields--[view-name]--[machine-name].html.twig if you want to be even more specific. 

and there you just do 

{{ fields.field_name.content }}




  For multi value fields, is there a way to print just one value within a field at a time?


{% set field_values = row._entity.get('field_name').getValue() %}
{% for field_value in field_values %}
    &lt;li&gt;{{ field_value['value'] }}&lt;/li&gt;
{% endfor %}



  By doing {{ fields.field_color_hex.content }}, I printed them all at
  once.


In your case:

{% set hexes = row._entity.get('field_color_hex').getValue() %}
{% for hex in hexes %}
    &lt;li&gt;{{ hex['value'] }}&lt;/li&gt;
{% endfor %}