Ghostflame Sliver

Strip the color off the entire tribe and the math on every color-based interaction shifts at once: protection from a color stops naming your Slivers, effects that count or care about colors lose their handle, edicts and pacts with a color clause slip past, and creatures barred from blocking by color are forced to block all the same. The ability is unconditional and global, sweeping in any Sliver under any controller's command, so it threads the tribe's signature shared-effect design while quietly rewriting what color-hosed removal can legally point at. The wrinkle is that colorless is not strictly defensive: a colorless creature is also nonblack, so the swarm becomes a legal mark for the broad "destroy target nonblack" effects it would otherwise dodge. That two-edged nature is the real story. Most Sliver lords bolt on a keyword or a stat to push the beatdown forward; this one trades in the tribe's currency of color itself, and whether the change shields your board or exposes it depends entirely on what spells the table is holding. It is the rare member of the swarm that contributes nothing to the attack step and everything to the question of which removal can name your creatures at all.