'Horse of another color' is the opposite expression from Shakespeare's 'a horse of that color' in the Twelfth Night Act 2 Scene 3 where Maria told Sir Toby Belch that her purpose was indeed a horse of that color which meant that Sir Toby Belch got the same idea as hers. So, 'horse of another color' is an idea different from the other.
To combine these sentences together to flow neatly and be grammatically correct you could say: Early airships such as zeppelins were filled with hydrogen, but today's blimps use helium instead. This is the correct answer out of your choices.
Your answer is: Early airships such as zeppelins were filled with hydrogen, but today's blimps use helium instead.
Have an amazing day and stay hopeful!
“Constance Culmington, he reflected to himself, was exactly the sort of woman who would buy an island and surround herself with mystery! Nodding his head in gentle approval of his logic, Mr. Justice Wargrave allowed his head to nod... He slept..." Chapter 1, pg. 3
