Calculating Color Differences between two RGB values; or two HSV values - objective-c

Question

I have an application in which a user selects an area on the screen to detect color. The color detection code works fine and it scans and stores both the RGB and HSV values.
So here is the flow of the program:
The user hits "Calibrate" in which the selected area of screen calibrates the current "white" values and stores them in the app. This does not have to necessarily by white; for instance the user could calibrate anything as white. But we are assuming they know what white is and calibrate correctly.
So a user gets a white strip of paper and calibrates the app for the values as such for RGB and HSV:
WR:208 WG:196 WB:187 WH:25.814 WS:.099194 WV:0.814201
Now the RGB of 208,196,187 obviously does not produce a "true" white but this is what the user has calibrated it for since the lighting cannot be perfect.
So.
Since I have the "Calibrated white"; I can now gain a second set of variables from the camera. However I want to know when a color passes a certain level of difference from the white.
So for instance; I take a strip of paper of varying degrees of purple. On one end the purple is indistinguishable from white, and as the length of the paper increases it gradually gets closer to a solid; deep purple.
Assuming the user has the device calibrated to the white values stated previously.
Let's say the user tests the strip and receives the new RGB and HSV values accordingly:
R:128 G:49: B:92 H:326 S:.66 V:.47
Now this is where I get stuck. Assuming I have retrieved both of these values; how can I scale the colors using their respective V's. And how can I accurately calculate if a color is say "20% different than the calibrated white" or something of the sort.
I did some simple comparisons like such:
NSLog(#"testing");
if(r <= whiteR*whiteV && g <= whiteG*whiteV && b <= whiteB*whiteV){
NSLog(#"TEST PASSED");
[testStatusLabel setText:[NSString stringWithFormat:#"PASS"]];
[testStatusLabel setTextColor:[UIColor greenColor]];
}
else{
[testStatusLabel setText:[NSString stringWithFormat:#"FAIL"]];
[testStatusLabel setTextColor:[UIColor redColor]];
}
but tests that shouldn't pass are passing; for instance when I calibrate a strip to white and then I test another white piece of paper it is passing sometimes...
Please ask for further clarification if need be.
Edit 1:
I found some pseudo-code here that may do the trick between to HEX (RGB) color codes; but it does not take into account different V (lightning brightness) values.
function color_meter(cwith, ccolor) {
if (!cwith && !ccolor) return;
var _cwith = (cwith.charAt(0)=="#") ? cwith.substring(1,7) : cwith;
var _ccolor = (ccolor.charAt(0)=="#") ? ccolor.substring(1,7) : ccolor;
var _r = parseInt(_cwith.substring(0,2), 16);
var _g = parseInt(_cwith.substring(2,4), 16);
var _b = parseInt(_cwith.substring(4,6), 16);
var __r = parseInt(_ccolor.substring(0,2), 16);
var __g = parseInt(_ccolor.substring(2,4), 16);
var __b = parseInt(_ccolor.substring(4,6), 16);
var p1 = (_r / 255) * 100;
var p2 = (_g / 255) * 100;
var p3 = (_b / 255) * 100;
var perc1 = Math.round((p1 + p2 + p3) / 3);
var p1 = (__r / 255) * 100;
var p2 = (__g / 255) * 100;
var p3 = (__b / 255) * 100;
var perc2 = Math.round((p1 + p2 + p3) / 3);
return Math.abs(perc1 - perc2);
}