I draw my own patterns so I don't have any guidance on how much of each fabric to buy. I used to (and sometimes still do) just sort of do a "common sense" guess - if the quilt top overall is going to need about 5 yards of fabric, and it's roughly half blue and half red, I'm going to buy something like 2-3/4 yard of each color. Maybe a bit more if the pieces are small - more to get "eaten" by seam allowances.
Now that I have EQ7, I will often use its fabric estimates instead, but I usually go a bit over. I like a healthy scrap bin so leftovers aren't a bad thing in my world, and I'd rather have too much than not quite enough.