Hi,
The shading is not different. It's the same everywhere. The problem is that you have that button for each product of the listing. And thus, the more products are being displayed, the more you have the button superposed, and thus the darker the shading gets as the shading of each button gets superposed.
I would recommend to remove the shading of the button all together in the CSS of your Joomla template.
The thing is that normally, that button should display for each element, not at the top and floating like you have it. It does that because of the CSS of your Joomla template which style the class "modal" as a popup while mootools uses that class for buttons.
If you change the .modal CSS of your template to div.modal, then it will apply that CSS to popups only, and the buttons will display under each product image as they are supposed to, but that might not be what you want.
But that means that when you tweak the CSS of the button in your template to lower it, you need to be careful to use a.modal and not just .modal so that your changes only apply to your buttons and not the popups.