The range of colours available is limited but includes metallics which are great for colouring realistic jewellery and machinery.
Here's the whale I'm working on in Lost Ocean, coloured with the Slicci pens. As you can see the needle-point nibs get right into the smallest of spaces and give vibrant, bold colour. Often, gel pens can be very wet with a thick line that isn't suitable for this kind of intricate work, which is where pens like the G-Tec C4 and Slicci come in very handy.