Color/palette cycling using GLSL (example in SFML)

cycle.frag

uniform sampler2D texture;
uniform sampler2D colorTable;
uniform float paletteIndex;

void main()
{
    vec2 pos = gl_TexCoord[0].xy;
    vec4 color = texture2D(texture, pos);
    vec2 index = vec2(color.r + paletteIndex, 0);
    vec4 indexedColor = texture2D(colorTable, index);
    gl_FragColor = indexedColor;      
}

main.cpp

#include <cstdlib>
#include <iostream>
#include <SFML/Graphics.hpp>
#include <SFML/System.hpp>
#include <SFML/Window.hpp>

int main(int argc, char *argv[]) {
    
    sf::RenderWindow window(sf::VideoMode(800, 600), "SFML/OpenGL color cycling effects");
    
    sf::Texture texture;
    if(!texture.loadFromFile("rock-abpp.png")) {
        return EXIT_FAILURE;
    }

    sf::Texture colorTableTexture;
    if(!colorTableTexture.loadFromFile("color-mappings.png")) {
        return EXIT_FAILURE;
    }
    
    sf::Sprite sprite(texture);
    sprite.setScale(10.0f, 10.0f);
    sprite.setPosition(
        (window.getSize().x / 2) - (sprite.getGlobalBounds().width / 2),
        (window.getSize().y / 2) - (sprite.getGlobalBounds().height / 2)
    );

    sf::Shader shader;
    shader.loadFromFile("blur.frag", sf::Shader::Fragment);

    int paletteIndex = 0;
    int numPalettes = 9;

    while (window.isOpen()) {
        sf::Event event;
        while (window.pollEvent(event)) {
            if (event.type == sf::Event::Closed) {
                window.close();
            }

            if( event.type == sf::Event::KeyPressed ) {
                if(event.key.code == sf::Keyboard::R) {
                    std::cout << "Reloading shader..." << std::endl;
                    shader.loadFromFile("cycle.frag", sf::Shader::Fragment);
                }

                // Move "left" through the palettes
                if(event.key.code == sf::Keyboard::Comma) {
                    paletteIndex--;
                }

                // Move "right" through the palettes
                if(event.key.code == sf::Keyboard::Period) {
                    paletteIndex++;
                }
            }
        }

        // Wrap around the palettes!
        if(paletteIndex >= numPalettes) {
            paletteIndex = 0;
        } else if(paletteIndex < 0) {
            paletteIndex = numPalettes - 1;
        }

        shader.setParameter("colorTable", colorTableTexture);
        shader.setParameter("paletteIndex", static_cast<float>(paletteIndex) * 6.0f/256.0f);

        window.clear(sf::Color::Magenta);
        sf::RenderStates states;
        states.shader = &shader;
        window.draw(sprite, states);
        window.display();
    }

    return EXIT_SUCCESS;
}

Do you have an example color table texture?