The code here uses formulas from the following web site: http://softpixel.com/~cwright/programming/colorspace/yuv/ Hopefully this person has some accurate formulas :). Basically you can plug numbers from 0 to 255 in the RGB side to mix colors and press the "rgb2yuv" button to see what it looks like and fill in the YUV textboxes with the YUV values that represent the RGB. Or you can go the other way plugging numbers into the YUV part and push the "yuv2rgb" button to see what RGB this would create and change the color of the box to show what this YUV combination would look like.
On a related note, I have example programs in C and SIMD Assembly for converting yuv images to rgb on my SSE Image Processing page.
Related Projects @mikekohn.net