Can two colors that look equally bright still make a line in your coloring page? In one controlled test, yes. A red and green boundary remained visible in Clean Outline, even though their calculated brightness values were almost equal.
We created a synthetic six hundred forty by four hundred eighty image with two flat color regions. Red was RGB two hundred, zero, zero. Green was zero, one hundred two, zero. Using the standard weighted brightness calculation, their values differ by less than one level out of two hundred fifty five.
We generated both the color image and a grayscale control with the same A four setting in Chrome. In the actual downloaded PNG, a narrow band around the color boundary contained twelve thousand four hundred dark pixels for the color version, and zero for the grayscale control.
This is one synthetic image, one style, one browser run. It does not predict every photograph or style. If two objects have similar brightness but different colors, inspect the generated outline before printing.