They're both going to be close but for different reasons. GL2/6: Black Coffee is going to be close because the blended color is the same natural brown as Raquel Welch's RL4/6.
However, GL4/8: Dark Chocolate is close because the base color is the same. However, the medium golden brown that Dark Chocolate is blended with will make it lighter than RL4/6. The base color for RL2/6 is darker than RL4/6, so it is going to be darker, but if you want to keep the color dark then this color would be the best option.
Here are Heather's review videos for each color:
RL4/6: Black Coffee: https://www.youtube.com/watch?v=90uhU58PTFM
GL2/6: Black Coffee: https://www.youtube.com/watch?v=VqeqouvFQhY
GL4/8: Dark Chocolate: https://www.youtube.com/watch?v=hFnBJ5svslk