It's not a Photos problem but an HTML issue. The correct color image file is named all-it-red-ls.jpg. However, the darker version that one clicks on is named call-it-red-ls-510x600.jpg. It's a different image file, not a thumbnail. You'll have to determine how the darker image is created and why the color is rendered darker or change the file to the same file that the popup image is.
You might ask your question in the iWeb forum as those who hang out there are website developers and will have more detail for you.
Currently you're are using the following code:
<div class="slide easyzoom" style="width: 510px; overflow: hidden; position: absolute; transform: matrix(1, 0, 0, 1, 510, 0); -webkit-backface-visibility: hidden; height: 600px;">
<a href="http://cavecloth.com/wp-content/uploads/2016/01/call-it-red-ls.jpg" title="call it red ls">
<img itemprop="image" src="http://cavecloth.com/wp-content/uploads/2016/01/call-it-red-ls-510x600.jpg" alt="call it red ls" title="call it red ls">
<div class="zoom-button large icon-expand tip-top hide-for-small tooltipstered"></div>
</a>
</div>
Change it to
<div class="slide easyzoom" style="width: 510px; overflow: hidden; position: absolute; transform: matrix(1, 0, 0, 1, 510, 0); -webkit-backface-visibility: hidden; height: 600px;">
<a href="http://cavecloth.com/wp-content/uploads/2016/01/call-it-red-ls.jpg" title="call it red ls">
<img itemprop="image" src="http://cavecloth.com/wp-content/uploads/2016/01/call-it-red-ls" alt="call it red ls" title="call it red ls">
<div class="zoom-button large icon-expand tip-top hide-for-small tooltipstered"></div>
</a>
</div>
and it will be as you want it to be.
