Simple Color Picker with glReadPixel

I'm trying to read the pixel at the user's cursor when they right click on a triangular gradient, but glReadPixel is only returning black. If I make it a flat color instead of a gradient, it works, so I don't know what's wrong.

Code:

glBegin(GL_TRIANGLES);
        glColor3f(1.0f, 0.0f, 0.0f);
        glVertex2i(0, 0);
        
        glColor3f(0.0f, 1.0f, 0.0f);
        glVertex2i(128, 0);
        
        glColor3f(0.0f, 0.0f, 1.0f);
        glVertex2i(64, 128);
    glEnd();
glColor3f(1.0f, 1.0f, 1.0f);
//This next part only happens when they right click, but I removed that stuff for now for simplicity. This happens right after the triangle is drawn, though
GLubyte pixel[4];
glReadPixels(mPos[0], cheight - mPos[1], 1, 1, GL_RGBA, GL_UNSIGNED_BYTE, pixel);

I have lighting and depth_test disabled. I'm using GL_Blend for the shade model and changing it to GL_Flat doesn't fix it. I also tried both glReadBuffer(GL_Front) and glReadBuffer(GL_Back). This project is using SDL_GL but I doubt that's the problem since flat colors get picked just fine.

The only way this will work is if you are using an orthographic projection matrix. If you want to do this in 3D you must fire a ray into 3 space based on the click position of the mouse. Then you can check for ray/triangle collisions in world space or local space. The only important part of this is that the ray and triangle must be in the same space. Once you determine which triangle was hit you can then find the barycentric coordinates of the hit and use these coordinates to address the texture that is being used on the primitive.