Scrollbar became black when tested on android device??

[RiRin]

Scrollbar works fine in editor but when I tested on android device.... the scrollbar has some problem
I made the scrollbar that I set forebackground and background.
I set it to scrolling to show "onlyIfNeed".
I found that the whole scrollbar became black color.
When I set the scrolling condition to "whenDragging", everything seems to be fine BUT....
when I hold my touch after dragging, the whole scrollbar became black...
Do I have to set some extra to avoid this ??

[ArenMook]

Eh? Whole scroll bar becomes black? I can't think of anything that would cause anything to go black. Scroll bars don't even need shaders to work.