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.
Hope this helped! 🙂