Gradient palette editor — #ff0000 to #000000

/* https://grabient.com/_fxJfxufwgfcQft-fsagCGgJdgFpgYtgkLgGK */

background: linear-gradient(90deg, #ff0000 0.000%, #de1536 16.667%, #a83c42 33.333%, #6a0020 50.000%, #250000 66.667%, #000000 83.333%, #000000 100.000%);

<svg xmlns="http://www.w3.org/2000/svg" width="800" height="400" viewBox="0 0 800 400" preserveAspectRatio="none">
            <!-- https://grabient.com/_fxJfxufwgfcQft-fsagCGgJdgFpgYtgkLgGK -->
            <defs>
              <linearGradient id="gradient" x1="0.000" y1="0.500" x2="1.000" y2="0.500">
        <stop offset="0.000" stop-color="#ff0000" /><stop offset="0.167" stop-color="#de1536" /><stop offset="0.333" stop-color="#a83c42" /><stop offset="0.500" stop-color="#6a0020" /><stop offset="0.667" stop-color="#250000" /><stop offset="0.833" stop-color="#000000" /><stop offset="1.000" stop-color="#000000" />
              </linearGradient>
            </defs>
            <rect x="0" y="0" width="800" height="400" fill="url(#gradient)" />
          </svg>

["#ff0000", "#de1536", "#a83c42", "#6a0020", "#250000", "#000000", "#000000"]

// https://grabient.com/_fxJfxufwgfcQft-fsagCGgJdgFpgYtgkLgGK
// cosine gradient palette — https://iquilezles.org/articles/palettes/
const float STEPS = 7.0;
const float ANGLE = 90.0;

vec3 palette(float t) {
    vec3 a = vec3(-0.951, -0.914, -0.992);
    vec3 b = vec3(-2.288, -1.154, -1.254);
    vec3 c = vec3(0.134, 0.605, 0.361);
    vec3 d = vec3(1.581, 2.315, 0.394);
    return a + b * cos(6.283185 * (c * t + d));
}

// N stops sampled from the palette, blended like the CSS gradient
vec3 stops(float t) {
    float n = STEPS - 1.0;
    float x = clamp(t, 0.0, 1.0) * n;
    return mix(palette(floor(x) / n), palette(min(floor(x) + 1.0, n) / n), fract(x));
}

void mainImage(out vec4 fragColor, in vec2 fragCoord) {
    vec2 p = fragCoord - 0.5 * iResolution.xy;
    // CSS angle: 0 = up, clockwise; span = CSS gradient-line length
    vec2 dir = vec2(sin(radians(ANGLE)), cos(radians(ANGLE)));
    float len = abs(iResolution.x * dir.x) + abs(iResolution.y * dir.y);
    float t = dot(p, dir) / len + 0.5;
    fragColor = vec4(stops(t), 1.0);
}

[
  [-0.951, -0.914, -0.992],
  [-2.288, -1.154, -1.254],
  [0.134, 0.605, 0.361],
  [1.581, 2.315, 0.394]
]