It could be your nutrients. A lot of lines using A & B color one so you can tell the difference. I was using House & Garden and their A has a redish tint to it. The roots would then take on a bit of the color. It never hurt the plants or effected the grow, they just weren't pure white. Your plant looks good, so I wouldn't be too concerned unless you start to see the roots turn brown or see slime forming on them. If you're running sterile you might want to take a look at a product from Cultured Solutions called UC Roots. It will prevent most common root issues and your roots will be pearly white.