How to make single color face for a cube in GLSL opegl

I have a rotating cube which continuously rotates

But it always rendered in such a way with interpolated gradient color

I want to color single face with only one color exactly one triangle piece as below

My GLSL

#version 410 core
attribute vec3 position;
attribute vec3 color;
varying vec3 outColor;
uniform float time;
uniform mat4 matrix;
void main()
{
    float theta = time;
    
    float co = cos(theta);
    float si = sin(theta);

    mat4 rotationY = mat4(co, 0, si, 0,
                          0, 1, 0, 0,
                         -si, 0, co, 0,
                         0, 0, 0, 1);

    mat4 rotationX = mat4(1, 0, 0, 0,
                          0, co, -si, 0,
                          0, si, co, 0,
                          0, 0, 0, 1);

    outColor = color;
    gl_Position = matrix * rotationY * rotationX * vec4(position,1.f);
}

#version 410 core
varying vec3 outColor;
uniform float time;
void main()
{
    gl_FragColor = vec4(outColor,1);
}

My GLFW code

        glClearColor(1f, 1f, 1f, 1f)
        glClear(GL_COLOR_BUFFER_BIT.toUInt() or GL_DEPTH_BUFFER_BIT.toUInt())
        
        var time = glfwGetTime()
        glUniform1f!!(uniformTime, time.toFloat())
        
        glDrawElements(GL_TRIANGLES, verticesIndex.size, GL_UNSIGNED_SHORT, NULL)
        
        glfwSwapBuffers(window)
        glfwPollEvents()

My color array

[
0.8, 0.007315, 0.026764, 
0.01696, 0.8, 0.005661, 
0.007843, 0.001524, 0.8, 
0.01696, 0.8, 0.005661, 
0.007843, 0.001524, 0.8, 
0.01696, 0.8, 0.005661, 
0.007843, 0.001524, 0.8, 
0.01696, 0.8, 0.005661, 
0.007843, 0.001524, 0.8, 
0.01696, 0.8, 0.005661, 
0.007843, 0.001524, 0.8, 
0.01696, 0.8, 0.005661
]

>Solution :

The usual way is to create a mesh with color attributes where each face has its own vertices with a color attribute of the same color.

Alternatively, you can try using the flat Interpolation qualifier:

The value will not be interpolated. The value given to the fragment shader is the value from the Provoking Vertex for that primitive.

Vertex shader

#version 410 core
in vec3 position;
in vec3 color;

flat out vec3 outColor;

void main()
{
    outColor = color

    // [...]
}

Fragment shader:

#version 410 core

flat in vec3 outColor;
out vec4 fragColor;

void main()
{
    fragColor = vec4(outColor, 1.0);
}

Another option would be to create an array of colors for each triangle primitive and store this color array in a Shader Storage Buffer Object. Use gl_VertexID to address the color in the vertex shader.

vec4 outColor;

layout(std430, binding = 0) buffer primitiveColors
{
    vec4 colors[];
};

void main()
{
    outColor = colors[gl_VertexID / 3];

    // [...]
}