What is the pin to light up colon in Basys 2?
I don't think the colon lights up. 

From reading the schematic, in page 2 you can see that there are 12 wires, none of which seem to drive the colon (there are wires CA..CG for the segments, DP for the decimal point and AN0..AN3 for multiplexing the displays).

Looking at the datasheet of the 7-segment display, does not have a way to light up the colon, either.

I worked once with a similar board (Nexys3) and I don't remember the colon lighting up.