Given an RGB value, how do I create a tint (or shade)?

I'm currently experimenting with canvas and pixels... I'm finding this logic works out for me better.

1. Use this to calculate the grey-ness ( luma ? )
2. but with both the existing value and the new 'tint' value
3. calculate the difference ( I found I did not need to multiply )

4. add to offset the 'tint' value

   var grey =  (r + g + b) / 3;    
   var grey2 = (new_r + new_g + new_b) / 3;
   
   var dr =  grey - grey2 * 1;    
   var dg =  grey - grey2 * 1    
   var db =  grey - grey2 * 1;  
   
   tint_r = new_r + dr;    
   tint_g = new_g + dg;   
   tint_b = new_b _ db;
   

or something like that...