Gradient with fixed number of levels

Question

I drawing a set of quads. For each quad I have a defined color in a vertex of it.

E.g. now my set of quads looks like:

Answer 1

My guess is there will be issues with the hue colours you're using and vertex interpolation (e.g. skipping some bands). Instead, maybe pass in a single channel value and calculate the hue and discrete levels (as @vesan does) within the fragment shader. I use these functions myself...

vec3 hueToRGB(float h)
{
    h = fract(h) * 6.0;
    vec3 rgb;
    rgb.r = clamp(abs(3.0 - h)-1.0, 0.0, 1.0);
    rgb.g = clamp(2.0 - abs(2.0 - h), 0.0, 1.0);
    rgb.b = clamp(2.0 - abs(4.0 - h), 0.0, 1.0);
    return rgb;
}

vec3 heat(float x)
{
    return hueToRGB(2.0/3.0-(2.0/3.0)*clamp(x,0.0,1.0));
}

and then

float discrete = floor(pass_Value * steps + 0.5) / steps; //0.5 to round
out_Color = vec4(heat(discrete), 1.0);

where in float in_Value is 0 to 1.