What are the rules on the collosal damage symbol? Does it modify damage value?

It's a big fist. That's about it.
Most Colossal figures have special rules that apply to playing them (check their rules booklet for details), but the symbol itself does little more than make them ineligble for certain feats.

While it makes them ineligible for certain feats, it also prevents them from gaining certain bonuses from certain feats [like Haymaker] and taking penalties from others [like Homing Device]. The special rules for most Colossal figures restrict feats further...Galactus and Spectre cannot be assigned them at all, while figures like Starro, Sentinel, and the Phoenixes cannot use feats during a Multiattack.