Replace specific color in HLSL shader

Question

I am trying to edit an existing shader so that it works only on 1 color and then make it so that this 1 color can get replaced with different one.

This is original code

sampler s0 : register(s0);
float4 p0 : register(c0);
float4 p1 : register(c1);

#define width (p0[0])
#define height (p0[1])
#define counter (p0[2])
#define clock (p0[3])
#define one_over_width (p1[0])
#define one_over_height (p1[1])

#define PI acos(-1)
#define cutoffR 0.40
#define cutoffG 0.025
#define cutoffB 0.25
#define cutoffY 0.25
#define acceptanceAmplification 5

#define showR 1
#define showG 1
#define showB 1
#define showY 1

float4 main(float2 tex : TEXCOORD0) : COLOR
{
    float4 color = tex2D(s0, tex);

    float luminance = (color.r + color.g + color.b)/3;
    float4 gray = {luminance,luminance,luminance, 1};

    float redness       = max ( min ( color.r - color.g , color.r - color.b ) / color.r , 0);
    float blueness      = max ( min ( color.b - color.r , color.b - color.g ) / color.b , 0);
    float greenness     = max ( min ( color.g - redness, color.g - blueness), 0);
    
    float rgLuminance = (color.r*1.4 + color.g*0.6)/2;
    float rgDiff = abs(color.r-color.g)*1.4;

    float yellowness = 0.1+rgLuminance*1.2 - color.b - rgDiff;

    float4 accept;
    accept.r  = showR * (redness - cutoffR);
    accept.g  = showG * (greenness - cutoffG);
    accept.b  = showB * (blueness - cutoffB);
    accept[3] = showY * (yellowness - cutoffY);

    float acceptance = max (accept.r, max(accept.g, max(accept.b, max(accept[3],0))));
    float modAcceptance = min (acceptance * acceptanceAmplification, 1);

    float4 result;
    result = modAcceptance * gray + (1.0-modAcceptance) * color;

    return result;
}

Source : github

I changed

#define cutoffR 0.40
#define cutoffG 0.025
#define cutoffB 0.25
#define cutoffY 0.25

to

#define cutoffR 1
#define cutoffG 1
#define clip(B) 0.25
#define cutoffY 1

So that it affects only blue color, as I am trying to turn blue into purple. Right now it is turning blue into gray and I don't know how to proceed further.

I changed code a bit

sampler s0 : register(s0);
float4 p0 : register(c0);
float4 p1 : register(c1);

#define width (p0[0])
#define height (p0[1])
#define counter (p0[2])
#define clock (p0[3])
#define one_over_width (p1[0])
#define one_over_height (p1[1])

#define PI acos(-1)
#define cutoffR 1
#define cutoffG 1
#define clip(B) 0.25
#define cutoffY 1
#define acceptanceAmplification 5

#define showR 1
#define showG 1
#define showB 1
#define showY 1

float4 main(float2 tex : TEXCOORD0) : COLOR
{
    float4 color = tex2D(s0, tex);

    float luminance = (color.r + color.g + color.b)/3;
    float4 color2 = (0);

    float redness       = max ( min ( color.r - color.g , color.r - color.b ) / color.r , 0);
    float blueness      = max ( min ( color.b - color.r , color.b - color.g ) / color.b , 0);
    float greenness     = max ( min ( color.g - redness, color.g - blueness), 0);
    
        


    float4 accept;
    accept.r  = showR * (redness - cutoffR);
    accept.g  = showG * (greenness - cutoffG);
    accept.b  = showB * (blueness - clip(B));


    float acceptance = max (accept.r, max(accept.g, max(accept.b,(0))));
    float modAcceptance = min (acceptance * acceptanceAmplification, 1);

    float4 result;
    result = modAcceptance * color2 + (1.0-modAcceptance) * color;

    return result;
}

Swapped

float rgDiff = abs(color.r-color.g)*1.4; 

to

float4 color2 = (0);

and

result = modAcceptance * gray + (1.0-modAcceptance) * color;

to

result = modAcceptance * color2 + (1.0-modAcceptance) * color;

I can now enter a value in color2 = () between 0-1 which changes color from black to white, but that's pretty much it. I don't know how to replace it to a specific color.