It seems by default Impress text uses Format > Character > Font effects > Font color: Automatic., which intelligently switches from black to white on a dark background. For example if you Insert > Media > Photo Album and choose "Add caption to each slide" then when you click to add a title in the semi-transparent dark bar, the text will be white.

However, if you save the resulting presentation as .pptx and then view it in Microsoft PowerPoint for Android, the text is always black (and thus illegible black on gray in photo captions).

I don't know if Microsoft PowerPoint supports automatic text color. Even if it doesn't, it's important that Impress try to do the right thing, or at least the documentation should mention the restriction.

FWIW the ODP XML <style:text-properties> attributes for Font color: Automatic might be
   style:use-window-font-color="true" loext:opacity="0%