Instead of
refPix.setRed ( (int) Math.round (refPix.getRed () * (1.0+ a) ));
Try something like
refPix.setRed ( (int) Math.round (refPix.getRed()*(1.0-a)+255*a ));
when a = 1.0, you get R*0.0+255*1.0 = 255
when a = 0.0, you get R*1.0+255*0.0 = R
when a = 0.5, you get R*0.5+255*0.5 (half half)
This works with any colour not just white, you just need to replace the 255 for red, green and blue with the colours of what colour you want to blend with it and you get RGB-averaged blending.