The song doesn't actually specify a colour, just how the girl he loves compares to the rose.
The yellow rose of Texas.
Yellow
conrós Éireannach
If you listen I'll sing you a sweet little song Of a flower that's now droped and dead, Yet dearer to me, yes than all of its mates, Though each holds aloft its proud head. Twas given to me by a girl...
The song is called Black Horse & the Cherry Tree, by KT Tunstall.