I need to make a 3 layer 8 inch cake with four shades/rows of pink rosettes. About how much bc do you think it would take to do one row of rosettes? I always over estimate and I don't want to color too much the wrong shade. Maybe 1 or 1.5 cups? I know most of you bake more often than me! Thanks 

An older rule of thumb was 1batch of icing using 2pounds sugar would fill, crumb coat, frost & decorate any size, shape cake made from 1 cake mix (or similar scratch recipe)   Some of those sizes are  10” round; 8” square, 12x8 sheet; each 2” deep for size comparison.   But I have not covered any cake with Rosettes therefore not sure how to advise you:(   

Leftover icing can be used for practice or fzn.  Or you can prepare for the need of black icing :)

Black is a combination of all colors so when you finish any b’cream project scrape all leftovers from bag, tip, bowl & save I a jar in the freezer.  No need to be concerned about how tiny the amount or color just freeze it together.  When needed  thaw the day before, mix well then add black powder or gel color mixing well& leave overnight for the color to develop.   If still not dark enough remove about 1/4 amount & MW it just till it starts to melt then return it to the full batch mixing well.  

I've only made a few cakes covered in rosettes and, believe me, it will take a whole lot more frosting than you might think.  At a guess, I was say roughly three times what an ordinary frosted cake would require.  Way too much icing for me!

When doing an ombre, it makes sense to start with the lightest color and add a little bit of color with each row of rosettes.  However, for me that is backwards, as I prefer the darkest color to be on the bottom.  Maybe you should try piping rosettes on an overturned cake pan to see how much frosting is required to go around the cake once, then divide into four portions of that size and tint them.  Hope that makes sense.