Error TypeError: Unsupported operand types: null + array...

Hola, Sergio. Welcome to the Backdrop CSM forum.

The first error is most likely related to the Color module -  I doubt this is related to the replacement of images. There must be something wrong with the color palette or color files placed inside the files folder.

It's tricky to figure this out just from your description. I would suggest you visit the Backdrop "office hours" to get help, or post the problem in the Zulip Backdrop channel where people can interact more or less in real time.

It would also be helpful to know which version of Backdrop you are using, and what theme.

Also, just to try, see if disabling and uninstalling the Color module helps. You'll lose all your customizations, so you want to do this not on the production site, and after backing up the database, files and config directories.  

For the second error, we need to figure out which theme hook is producing it. Are you able to enable Devel and insert a debuging line where the problem happens?

In line 1102 of theme.inc, try replacing:

  if (!empty($info['variables'])) {
    $variables += $info['variables'];
  }

with:

  if (!empty($info['variables'])) {
    if (is_null($variables) {
      dpm($hook);
      $variables = array();
    }
    $variables += $info['variables'];
  }

This may produce other problems, but at least we'll know which theme hook is producing the issue.