VESA is a drastical downgrade of your video card. Check first, which mode SNAP uses (it is displayed on the front page of the "Screen notebook").
If it says "VESA VBE 3.0" or something like that then SNAP cannot operate your video card and uses a standard tool (VESA) for it. In this case, the 256 colors can be imposed by the VESA abilities of your video card.
If it identifies your video card correctly then you should consult Scitechsoft because something is wrong with their driver. The driver should offer more colors and more colors should be also displayed.
In case of VESA, download the "real SNAP" and check whether it will work. Keep in mind that the SNAP, which you have got free of charge with your eCS, is a restricted version, which does not support all cards supported by the real SNAP and which does not support all abilities of the video cards. Moreover, the "free SNAP" on your CDs can be old.
The real SNAP "demo" (which you will download) is a fully functional full version, which will run for 21 days. During this period, you will need to decide, whether you want to buy it or whether you uninstall it.
If the real SNAP won't work either, then you are out of luck currently. But don't lose hope. Some further version of SNAP can be familiar with your video card. This happened to me with my GeForce 4 Ti 4200 8x as well