I just opened up a configurable product and saw that when I reset the filter for the list of associated products that some of the products have a red background. I never saw that before. Is there something broken? Thanks for your help!
This is normal behavior.
You should notice that the checkboxes on the red lines are disabled.
This means that you cannot associate those products to the main configurable product.
Here is why.
Let's take an example. You have a configurable product with configurable attributes Color (Red and Blue) and Size (S and XL).
You can have for this configurable product max 4 simple products associated:
- Red S
- Red XL
- Blue S
- Blue XL
Let's say you already assined
Red S to the configurable product but in the same attribute set as the configurable product you have other simple products with the combination
Red S. (see here an explanation why there is a restriction to the same attribute set)
You cannot assign an other
Red S product to the configurable product unless you un-assign the one that's already assigned.
So the simple products that are allowed be added as associated to the configurable product but cannot be added because you already have that attribute combination set, appear in red background and with the checkbox disabled.