Yea, stick with whats in them. But be wise about it. For example, If I need to make a light lime green, I start with one full marker of white as a base (very important, and sometimes I pour in more white to fill the marker completely so that I have enough to get the job done) and add a tiny bit of yellow to make it pale pale yellow, and then a microbe of blue to switch it to the green.
These paints do not behave at all like normal acrylics, so be aware of that.
Watering down acrylic to put inside the marker will not work, it will just make a mess of your canvas.
That said, here's a piece i did with 95% custom mixed colours. Its on a long board.
Sort of a grainy picture, the only colours I didnt mix, are the white and orange. And thats a dark purple you see, not black or whatever....
Letters kind of suck on this one...