I prefer the ones with the purple stripe down the side. Mostly cause my romper has a random purple design on the chest that would kinda be weird to be the only place with purple, and because purple is awesome.
It's pretty easy to add to an existing pattern, but just in case here is how you draft the pattern. I used McCalls 7026, because I can't find the tights pattern I drafted right now. At least I used this for a store display when I still worked in a fabric store, so I know they fit.
So cut out the tights pattern to whatever size you are and the trace that sucker out (if you are going to reuse the pattern later, if you are never going to reuse it, you can probably just cut up the original).
Find the center of the side of your legging. Legging/tights patterns come with the front and back as one piece. I just held it up to myself and marked it in the center to do it. You can also just eyeball it on the flat pattern.
Once you have found the center, draw a line from the waist to the cuff as a guideline.
Decide how big you want your stripe to be, divide that by 2 and add that on to each side of that center line. I chose to make it easy on myself and just did 2 inches.
Do that all the way down.
Then label your pattern pieces, cut apart.
IMPORTANT: once you have done this, remember that the seams where you attach the stripe to the rest of the leggings DOES NOT have seam allowances!! So do not forget to either add that to your pattern, or make sure when you cut it out you add them back in, or they will be too tight. I left 1/4 inch seam allowances on all 3 pieces when I cut them out.