Jetpack Premium, Tiled Galleries, Layout, Slideshows

Sorry for the confusion; you’re actually mentioning 3 different features, that behave differently from one another. Let me try to clarify:

• Jetpack’s Tiled Galleries change the look of your regular WordPress galleries: the gallery images will have a different layout, but there is no background color involved there.
• Jetpack’s Carousel appears when you click on an image inside a WordPress gallery. It looks like this:
  https://jetpackme.files.wordpress.com/2012/07/carousel-black-screenshot.png
  By default, the carousel view has a black background. You can change that background to white by going to Settings > Media in your dashboard. Here is how things look like when using the white background option, and clicking on an image inside a gallery:
  https://jetpackme.files.wordpress.com/2012/07/carousel-white-screenshot.png
• Jetpack’s Slideshow feature becomes available when you enable Jetpack’s Shortcode embeds module. It allows you to replace a WordPress gallery by a slideshow of images. All you will see is a single image and buttons allowing you to move through the slideshow. Here is how it looks:
  https://i.wpne.ws/ike3
  That slideshow background is back by default. While there are no options to change the background color of these slideshows, you can customize the look of every element on your site thanks to CSS. You can add that CSS to your theme stylesheet, or to your custom CSS editor available under Appearance > Edit CSS in your dashboard. You could for example switch the slideshow background color to transparent, so it blends in the page, thanks to the following CSS:

  .slideshow-window {
  	background-color: transparent;
  	border-color: transparent;
  	border-radius: 0;
  	border-width: 0;
  	padding-top: 0 !important;
  }

I hope this clarifies things a bit!