The configuration options you posted do not include any Back Button options but if I add the following to a Juicebox-Pro v1.4.0 test gallery using your configuration options, the Back Button appears black on a white background.

backButtonPosition="TOP"
backButtonUseIcon="TRUE"

It sounds like you might be using an older version of Juicebox-Pro (although if you just downloaded it yesterday, this is unlikely) or there may be a conflict with some custom CSS code on your web page (if you have embedded your gallery into an existing web page alongside other content).
Please check for these two possibilities and if you continue to experience difficulties, please post the URL to your gallery so that I can take a look and help further.
(You can check which version of Juicebox-Pro your gallery is using by looking at the comments at the top of the 'jbcore/juicebox.js' file and you should be able to see if there are any custom CSS rules being applied to the Back Button using your browser's developer tools.)

The configuration options you posted above are not the same as the ones used in the gallery whose URL you posted. The gallery on your web page uses the following configuration options.

gallerywidth="100%"
galleryheight="100%"
backgroundcolor="#ffffff"
textcolor="rgba(255,255,255,1)"
thumbframecolor="rgba(255,255,255,.5)"
showopenbutton="TRUE"
showexpandbutton="TRUE"
showthumbsbutton="TRUE"
usethumbdots="FALSE"
usefullscreenexpand="FALSE"
screenmode="SMALL"
imageshadowcolor="rgba(255,255,255,255.1)"
showautoplaybutton="TRUE"
shownavbuttons="TRUE"

The gallery on your web page sets screenmode="SMALL".
If you want to use the Back Button in Small Screen Mode, you will have to set showSmallBackButton="TRUE".
Also, please note that buttonBarBackColor (the background color for the Button Bar) is a Large Screen Mode configuration option only. (This is noted in the description for buttonBarBackColor on the configuration options page.) When you gallery is displayed in Small Screen Mode, the default value of rgba(0,0,0,.5) will be used for the buttonBarBackColor.

If you want to use a Spage Page but want your gallery to be displayed in Large Screen Mode wherever possible, set screenMode="AUTO" and showSplashPage="ALWAYS" (instead of screenMode="SMALL").

I do not see a Juicebox gallery on the web page whose link you posted (http://www.reinaertdevos.eu/content/venlo-1) although there is embedding code on the page.
The settings used in this gallery can be seen here: http://www.reinaertdevos.eu/juicebox/xm … es/default
These settings result in a white background for both the Button Bar and the Back Button (in Large Screen Mode).

However, the settings used in the 'teaser' gallery on this page: http://www.reinaertdevos.eu/
... are quite different: http://www.reinaertdevos.eu/juicebox/xm … ges/teaser
... and, because screenMode="SMALL" is used, the background for the Button Bar is the default value of 'rgba(0,0,0,.5)'.

It looks like you might want to change the settings in the 'teaser' gallery so that screenMode="AUTO" and showSplashPage="ALWAYS".

That is correct. When I click on the 'Venlo 1' link on your home page, I see no gallery on the resulting web page. (I have viewed the web page in Firefox 28.0, Chrome 34, IE11, Opera 20.0 and Safari 5.1.7.)
It looks like the CSS on your web page may be forcing the gallery container's position and/or size to be such that the gallery itself is not visible on screen.

With regard to your original query, we now plan to open up buttonBarBackColor and captionBackColor for use in Small Screen Mode as well as Large Screen Mode in a future version of Juicebox-Pro.

I have run your web page through the W3C Markup Validation Service and notice that there are two unclosed <div> elements on your web page. I do not know what part of your Drupal installation is responsible for this (Drupal core, a theme, module or custom code) but investigating this and fixing the HTML errors on your web page would be the first thing I would do. (Try using a different theme to see if your current theme is the source of the problem.)
Once the code on your web page validates correctly, your gallery should hopefully display correctly.
Even if fixing the HTML errors on your web page does not fix the problem, we will at least be able to eliminate them as a possible cause.

Also, it looks like your gallery may somehow have a width of zero.
If I give the <div class="juicebox-parent"> container on your web page a fixed width such as 800px (using Firefox's developer tools), your gallery seems to display OK.
Perhaps you could post your query in the Drupal forum. Please note that we did not write the Juicebox module for Drupal ourselves. If you post your query in the Drupal forum, the author of the module may be familiar with the problem and be able to offer a solution.

I hope this helps and points you in the right direction.