"Colossal flashes inside her" refers to the effect of the British heavy shells hitting the German battleship. The word "colossal" indicates that the explosions caused by the shells were of great size and force, while "inside her" suggests that the flashes were happening within the battleship itself, likely causing significant damage and destruction. The phrase conveys a sense of the intense violence of the battle and the devastating impact of the British attack on the German ship.