code: public void setSeparatorColor(int separatorColor) {
          mSeparatorColor = separatorColor;
  
          Field[] pickerFields = NumberPicker.class.getDeclaredFields();
          for (Field pf : pickerFields) {
              if (pf.getName().equals("mSelectionDivider")) {
                  pf.setAccessible(true);
                  try {
                      pf.set(this, new ColorDrawable(separatorColor));
                  } catch (IllegalAccessException | IllegalArgumentException e) {
                      e.printStackTrace();
                  }
                  break;
              }
          }
      }
docs: Uses reflection to access divider private attribute and override its color
  Use Color.Transparent if you wish to hide them