Default Uniform Values

In GLSL 1.10 all uniforms are assigned a default value of 0 when a program is successfully linked.  In GLSL 1.20 all uniforms, except for samplers, can have an initializer, like so:

uniform vec3 color = vec3(0.7, 0.7, 0.2);

All uniforms without initializers are assigned a default value of 0.

References:

GLSL 1.20 specification, p. 24