Try this in shopify theme.liquid or index.liquid or any other liquid wherever the class="site-header"
{% if shop.url == 'http://wetpaintrecords.com' or shop.url == 'wetpaintrecords.com' %}
<div ... class="site-header">
.....
</div>
{% endif %}
Alternatively add this anywhere in your theme.liquid file
{% if shop.url == 'http://fetchy.ca' or shop.url == 'fetchy.ca' %}
<style>
.site-header{display:none !important;}
</style>
{% endif %}
EDIT: OP is using an iframe/frameset/frame, hence the following solution.
You have two options here.
Option 1 (simplest) - Add the following in your theme file liquid:
{% unless collection %}
<style>
.site-header{display:none !important;}
</style>
{% endunless %}
Option 2 (not simplest but safest) - Find the liquid file containing 'header class="site-header" role="banner"' and change that section to the following:
{% unless collection %}
<header class="site-header" role="banner">
..........
</header>
{% endunless %}
By this, whenever the page is a collection (artist page as intended), the section will not be displayed.