How to change this shader to swap the palette

[Heavybrush]

Hi, I created a shader that get the actual color of the surface and render it to the closer of a cga palette
I have to change it to swap the palette in 4 colors but I don't know how
secondary, I have to render it dynamic (maybe using an uniform variable)
because it have to change in base of the palette
please help me, thanks

///CGA Shader
varying vec2 v_vTexcoord; //xy
varying vec4 v_vColour; //rgba

#define _ 0.0
#define o (1./3.)
#define b (2./3.)
#define B 1.0

#define check(r,g,b) color=vec4(r,g,b,0.); dist = distance(sample,color);if (dist < bestDistance) {bestDistance = dist; bestColor = color;}

void main(out vec4 v_vColour, in vec2 v_vTexcoord)
{
float dist;
float bestDistance = 1000.;
vec4 color;
vec4 bestColor;

vec2 pixel = vec2(320,200)-floor( v_vTexcoord.xy / iResolution.xy * vec2(320,200));
vec4 sample = texture2D(iChannel0, pixel / vec2(320,200));

sample += (texture2D(iChannel1, pixel / iChannelResolution[1].xy)-0.5)*0.5;

// pallete 0

check(_,_,_);
check(o,B,o);
check(B,o,o);
check(B,B,o);

vec4 color1 = bestColor;
bestDistance = 1000.;

// pallete 1

check(_,_,_);
check(o,B,B);
check(B,o,B);
check(B,B,B);

vec4 color2 = bestColor;

float t = (clamp(sin(iGlobalTime)*4.,-1.,1.)+1.)/2.;

v_vColour = mix(color2, color1, t);

gl_FragColor = v_vColour * texture2D( gm_BaseTexture, v_vTexcoord );
}