If I understand what you want correctly, I think you can have that section transparent and place the image ABOVE the color indicator. That way, the control will be seen through the transparent area. I don't think JPEG supports transparency, and LV itself may be a bit hard on the subject, so you may have to play around with it.
I'm not sure what you mean by percentage - do you want the area to have both red and white at the same time, or do you want to control the amount of white in the color. If you want two colors, I think you will need to use a slide.
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