I want to make a cocktail which uses Yellow Chartreuse. I happen to have a quarter-bottle of Green Chartreuse in the back of the cabinet. I know the green variety is stronger and the yellow somewhat sweeter. I'm thinking I'll use a little less, increase the (other) sweeter ingredients, and maybe pick a stronger base liquor to balance better. Will this result in something mostly like the original cocktail, or will it be more like a whole new thing (for better or worse)?
You cannot substitute green for yellow Chartreuse without changing the basic character of the cocktail. Both varieties are very strong flavors, but different characters. As the Chartreuse is nearly 25% of the ingredients you can't hide the difference.
If you make a Green Jacket instead of a Yellow Jacket you'll end up with something more vegetable tasting and less sweet. It may taste good or it may not, there's no way to predict. You could try dropping the green Chartreuse to 1/2 an ounce and adding a dash of honey, that may get you closer. If you do use honey try mixing it in with the Chartreuse before adding it to the cocktail - you want it well blended.