Bromothymol blue is an acid base indicator. When the solution is basic it is a blue color. As the pH descends to neutral it turns sea green as it passed through a pH of 7.0, and when it crosses over into the acidic range it becomes a light yellow color. The color sequence can be reversed by slowly adding bases to the solution.
LL Demos & Lab: Bubbly Blue Breath, Technicolor Fog Factory