When to switch to different color yarn/pattern help

[splent2112]

I'm making an amigurumi and am following this pattern. I finished the top ball of it, but am stuck on rnd 19, where it says slip stitch in each sc. Also, at some point I'm supposed to switch to light blue yarn, but it's not clear where I'm supposed to do it. Am I supposed to be doing that in rnd 19, or in a minute (the rnd 20 step is confusing as well... once I'm past that I should be fine).

Here is the pattern I'm following:

https://www.miahandcrafter.com/atelier/piplup-pattern/

And attached is how far I am. I'm done with rnd 18. 

[Granny Square]

Welcome to the 'ville!  

Glad I took a few years of Spanish eons ago, the designer slipped up in row 8 and said 'em cada' instead of 'in each'.  (I know that's Portugese, but close enough).

Looking at the photo of a dark blue ball attached to a light blue one, I am going to guess the light blue happens at row 20, just by the fact that you start with dark blue, increase and then decrease down to 12 (which would make a ball shape, round 19 is a round of slip stitches instead of SC, and row 20 says "Crochet Rnd 20 as if rnd 19 and 18 were one (as if each slip slitch were a part of each sc)" - which is unusual wording but sounds like a joining round to me.  I'm not sure why they did it that way, I've never seen this instruction before when changing colors, maybe it makes a smoother color transition?

[splent2112]

Trying it and it seems to be working. I just hope I joined it correctly... it seems secure, I'll just have to weave in the extra of the top yarn later.