In most of my patterns, especially me new ones I change the color in the last st mentioned.
So for example:
If it says With color A, 1 sc in each of next 3 sc, Switch to Color B in last st, 1 sc in each of next 10 sc = I will start by working the first 2 sc out of 3, and on the 3rd sc I will switch the color (when having two loops on the hook I will yarn over with new color). In result the 3rd sc will stay in color A and the 10 sc to follow will be in color B.
Work first 2 sc.
Insert hook into next sc (last st mentioned in color A= 3rd sc).
Yarn over and pull yarn through. (2 loops on hook)
Yarn over with new color (color B).
Pull new color through two loops on hook.
Work 10 sc. 
Hope this helped! 🙂
