If you install the TabhostExtras library in the additional libraries path and reference it, you can change the color of the tabhost indicator. Make sure you search in the forum for TabhostExtras to find the zip file that has the libray (jar and xml files). Here is a sample. You can use any RGB color combination you want:
B4X:
Dim TabHost1 As TabHost
Dim MyTabHostExtra As TabHostExtras
MyTabHostExtra.setTabGradientDrawable(TabHost1,"LEFT_RIGHT",Colors.RGB(167,0,80),Colors.RGB(95,158,160),6)