Transfering 2.X code to a physically based shader

I've got a 2.X shaderlying around that uses an RGB image to change the tint of different parts of the diffuse map, I would like to make it an additional option in the Standard Shader but I'm assuming I can't just drag and drop the code and expect it to work. Any tips on upgrading it? I've included the relevant code section below.

Code (csharp):

1.    float4 texcol = tex2D( _MainTex, i.uvK.xy ); //samples the main diffuse texture
2.  
3. // ignore black
4. float4 c = tex2D( _ColorChangeMap, i.uvK.xy ); //samples the RGB texture
5. if ((c.r==0) && (c.g==0) && (c.b==0))  // checks if all black (this is probably a bad way to check for it)
6. { /*nothing*/ }
7. else {    
8.     float4 cout = c.r*_RedColor + c.g*_GreenColor + c.b*_BlueColor;  // if not all black, multiply the base RBG user texture by custom set user colors in the properties
9.     texcol = texcol * cout; // multiply the modified color by the original diffuse map
10. }