How do I choose what horse I spawn?

The full list of mob attributes (including horse-specific attributes) can be found on the wiki. For your particular example, you need 'Variant' and 'Health', e.g.:

{Variant:515,Health:28,Attributes:[{Name:generic.maxHealth,Base:28}],Tame:1,Saddle:true}

Health is current number of half-hearts. The maxHealth attribute is the max number of half-hearts.

Variant is calculated as base-color-id + markings-id * 256, that is, 515 = 2 * 256 + 3 means base-color-id = 3 and markings-id = 2