Patterns often say "you MUST swatch" and "Swatching REQUIRED." You knit up a 4x4 inch square in whatever pattern they tell you to, and then you measure it to see if it matches what they say it should match (e.g. 7.5 st/inch or something like that).

I have way too many projects right now, but I'm gonna tuck that shades of red idea into the back of my mind for later....