Red was the color of Tadashi's favorite moped.

Red was the color of Hiro favorite shirt.

Red was the color of fire and flames.

Red was the color of Hiro's mourning.

Red was the color of Baymax's case.

Red was the color of the streaks on the Yokai's mask.

Red was the color of Baymax's armor.

Red was the color of sunset and San Fransokyo's Golden Gate Bridge.

Red was the color of Hiro's rage.

Red was the color of the Silent Sparrow project logo.

Red was the color of Hiro's revenge.

Red was the color of Tadashi's t-shirt when Baymax succeeded.

Red was the color of Baymax's fist.

Red was the color of the shirt Hiro wore when Baymax opens his eyes.

Red was the color of them.

Author's note :: This was really random, but after I watched the movie for the seventh time today, my theory was sorta confirmed.

The color red was very significant in this movie. Hiro wears red, but only in the presence of his brother or Baymax. The one time in the movie Baymax and Tadashi's gone, Hiro wears all blue, hiding the red underneath. Red, like said above, is the color of Tadashi's moped, also the color of Baymax's armor. Both were used to save and protect Hiro in their own way.

Of course, everyone remembers the destructive Baymax scene. Everyone also remember the significant red of Baymax's destructive ship. The red on the Yokai's mask is symbolic, the fire is red, the project logo is red, and yes, Tadashi wears red when he succeeded in finishing Baymax (that's the only time he wears it too). So my theory is that red is the color to tie the three of them (Hiro, Tadashi and Baymax) together. Take it as you will XD