Rotating triangle & changing its color in opengl - c++

Question

I'm new to openGL, I need help with animate a triangle that rotates 1 degree every 25 milliseconds. I want to program this triangle to gradually change color from blue to green to red.
float rAngle=0.0;
void handleResize(int w,int h)
{
glViewport(0,0,w,h);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluPerspective(45.0,(double)w/(double)h,1.0,200.0);
}
void drawscene()
{
glColor3f(0.0f,1.0f,0.0f);
glClear(GL_COLOR_BUFFER_BIT);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glRotatef(-rAngle,0.0f,0.0f,0.1f);
glBegin(GL_TRIANGLES);
glVertex3f(-0.5f,0.5f,-5.0f);
glVertex3f(-1.0f,1.5f,-5.0f);
glVertex3f(-1.5f,0.5f,-5.0f);
glEnd();
glFlush();
}
void update(int value)
{
rAngle+=1.0f;
glutPostRedisplay();
glutTimerFunc(25,update,0);
}
int main(int argc, char** argv)
{
glutInit(&argc, argv);
glClearColor(0.0f, 0.0f, 0.0f, 1.0f);
glutInitWindowSize(500, 500);
glutInitWindowPosition(0, 0);
glutCreateWindow("OpenGL");
glutDisplayFunc(drawscene);
glutReshapeFunc(handleResize);
glutTimerFunc(25,update,0);
glutMainLoop();
return 0;
}

Answer 1

This way of using OpenGL is deprecated.
Let me start off by pointing out that there is no reason to dabble with old outdated OpenGL. Immediate mode has been deprecated for about 10 years now.
Learn modern OpenGL instead.
Although the question is very unclear, I will try to give you the info you need.
Push/Pop Matrix:
glPushMatrix():
There is a stack of matrices for each of the matrix modes. In GL_MODELVIEW mode, the stack depth is at least 32. In the other modes, GL_COLOR, GL_PROJECTION, and GL_TEXTURE, the depth is at least 2. The current matrix in any mode is the matrix on the top of the stack for that mode.
glPushMatrix pushes the current matrix stack down by one, duplicating the current matrix. That is, after a glPushMatrix call, the matrix on top of the stack is identical to the one below it.
glPopMatrix pops the current matrix stack, replacing the current matrix with the one below it on the stack.
Initially, each of the stacks contains one matrix, an identity matrix.
Rotation:
To rotate the current matrix, call glRotate().
Color:
Use these to select rendering color.
Usually, glColor3f() or glColor4f() is used.
Your scenario:
Call glColor3f() or glColor4f() to select rendering color.
Call glPushMatrix() to avoid rotating everything.
Call glRotate() to rotate the matrix.
Render mesh.
Call glPopMatrix() to revert out of the rotated matrix.
To modify per time unit:
Just modify the data sent into the GL functions over time. Increase the rotation/color values a bit each frame.
I suggest reading up on matrix math, and how graphics incorporates this.