When you're done shaping the upper area of the bottle to the top of the bottleneck, hit ENTER. You'll see two red openings (circles), one at the top, the other at the bottom of the extruded surface. Amapi allows you to select which of these should be closed. You can click both or just the bottom to leave the top open. If you hit ENTER, both (all) openings are being closed. If you just swipe away, none are closed.

Click the bottom circle and swipe away to leave the top of the bottle open.

If you forgot to close it, there's a tool in the Tools > Advanced Tools menu to close an object.

Select the Render Settings (CTRL-J) from the Render menu, and click on the Background Color bar (which is black by default). Select a light blue color, or whichever background color you want. This will be the background color used during rendering.

While you're in the Render Settings, also check (enable) the Extended Editor option for the Material Editor section.

Click OK when done to validate these changes to the rendering options.