Android : Android: ImageView Background Color and Alpha

For an ImageView display, I would like to set that the users can set the background color and the alpha. Therefore, I have coded as follows after saving there preference:

if (k==1) {display.setBackgroundColor(getResources().getColor(R.color.tran_red));}
display.setAlpha( (int) (display_alpha_integer * 255 /100 ));

The alpha can be adjusted by a seekbar using the variable display_alpha_integer, which is working properly. Yet while the backgroundcolor is shown properly, the display alpha simply does not change when moving the seekbar.

How could the above be modified?