Make a control object with instance var or a global var (for example color=#) and set it to a different number for each color.
Now somewhere throw up a random number generator that constantly runs.. such as Every X seconds set instance var color to choose(0,1,2,3,4,5) or int(random((0,5)).
Now in your spawner, sub-event down for each color:
(main)Spawn event condition
(sub) instance color=0 - (action) spawn blue bubble
(sub) instance color=1 - (action) spawn red bubble
You could also use Every tick and it will race through 0-5 but your spawn event will still grab whatever current value and use it.
You could also use a Function that chooses and sets the color variable when called and call it in your spawning event and sub to the actual spawn event after.