how do I pass color info to vertex shader?

Ok, so I’m making fireworks and it works rather well.
As a basis for this I am using sparks, which are rendered as follows:

void Fireworks::render()
	//set the size of the rendering area
	glViewport(0, 0, width, height);
	//modelview transform matrix
	Matrix4 modelviewmatrix = Matrix4::lookAtMatrix(viewpoint,
	                                                Vector3(0.0f, 0.0f, 1.0f));
	//projection transform matrix
	Matrix4 projectionmatrix = Matrix4::perspectiveMatrix((float)M_PI_4, (float)width / height, 1.0f, 10000.0f);

	glClearColor(0.0f, 0.0f, 0.0f, 0.0f);

	//use the terrain shader
	//set our matrices as "uniform" variables
	glUniformMatrix4fv(glGetUniformLocation(terrainshader.getProgram(), "matmodelview"), 1, GL_TRUE, modelviewmatrix.elements());
	glUniformMatrix4fv(glGetUniformLocation(terrainshader.getProgram(), "matprojection"), 1, GL_TRUE, projectionmatrix.elements());
	spark* p = sparks;
	while(p != NULL){
	  for(int i = 0; i < 3; i++){
	    glVertex3f(p->position.x(), p->position.y(), p->position.z());
	    glVertex3f(20.0f*sin(rand())+p->position.x(), 20.0f*cos(rand())+p->position.y(), 20.0f*cos(rand())+p->position.z());
 	  p = p->next;

These go through the vertex shader which is

out vec3 _color;

void main()
	// Get a copy of the normal vector
	_color = ?
	// Transform the vertex position to screencoordinates
	gl_Position = matprojection * (matmodelview * in_position);

and the fragment shader which is

in vec3 _color; //normal vector

void main()
	//severely simple lighting model
 	fragcolor = vec4(_color , 1.0);

now I think I got the passing from the vertex to the fragment shader right, but I want to know how I can pass the color information from my program to the vertex shader. I can’t seem to find how to do that…

I could post the rest of the code, but I don’t think it’ll matter (and it’s probably bad anyway). I currently set fragcolor to a fixed color and it works.

From the snippet you have posted, it appears you want to assign the per-vertex normal as color. To do that, you would have to pass the per-vertex normals (glNormal3f) along with positions (glVertex3f). Assuming that the normal is passed in to vertex shader as in_normal, then you can simply assign it to the color output variable like this,

in vec3 in_normal;
in vec4 in_position;
out vec3 _color;

uniform mat4 MVP; //combined modelview/projection matrix

void main()
   // Assign the normal vector
   _color = in_normal;
   // Transform the vertex position to clip space
   gl_Position = MVP * in_position;

See if this helps.