If you have a Uni-Ball Signo TSI gel rollerball then these are the replacement inks that you need, with a 0.7mm medium tip they are available in Blue, Black and Pink:
If you have a Uni-Ball Fanthom Gel Pen then these are the ink replacements that you need, this is the medium 0.7mm and they are available in Black, Green, Sky Blue, Blue and Pink: