I can\'t find a way to change the color of the highlighted selection in the autocomplete box in Eclipse.
Alternatively you can update the variables at the top of the gtkrc file. For your particular problem it would be selected_fg_color, not tooltip_fg_color, for example, change it from white to black(#000000). Here are my settings:
gtk-color-scheme = "base_color:#ffffff\nfg_color:#4c4c4c\ntooltip_fg_color:#000000\n
selected_bg_color:#f07746\nselected_fg_color:#000000\n
text_color:#3C3C3C\nbg_color:#F2F1F0\ntooltip_bg_color:#FFFFFF\nlink_color:#DD4814"