I mean if I look at this pic of yours, it seems pretty obvious to me that there's a color mismatch:
http://i.imgur.com/jhdwpGW.jpg?1
Use normal Hair Color, I use this one on my system to match my sides:
https://www.amazon.com/LOreal-Paris-Excellence-Natural-Packaging/dp/B000GCQXO4?th=1
But a fair warning, color will stain the base if you touch it and hair system hair absorb more color, so for your first try leave the color for not more than 10 minutes. It's best to search Youtube for tutorials as there are various videos were people color their hair systems in different ways. Also when you do hair color, color your sides as well, this gives the best results but be aware that hair color can damage your natural hair.
The way I personally do it (and I don't recommend it, haha) is that I attach the piece like I normally do and then I color using a tooth brush and comb. This is risky as the piece is attached to my head firmly and so if I were to stain the front then I have no way to immediately clean it up.
