The plugs are different colors because some are running rich and some lean...causing a misfire...Pull the plug wires out of the cap one at a time with the engine running to see which ones don't cause an idle drop. Maybe pull two right next to each other and alternate them (connect/disconnect). Then remove the plugs keeping them in order, and see if the ones with the whitest insulators are from the cylinders that didn't cause an idle drop. And check the cap to make sure nobody cracked it against the firewall when they removed the trans.