Adding a new color to a scarf

:thinking: Hi,
I’m starting to knit again after 20 odd years, so there’s a lot I’ve forgotten. I dediced to start with something simple, like a scarf. I am knitting in a garter stitch and tried to add a new color. I proceeded to tie on the new color yarn at the end of the row and continued knitting. After knitting a few rows, I noticed the front of the scarf had a clean line where the two colors met, but the back of the scarf had a dotted look. The new color yarn had a line under the main color instead of a complete row of the contrasting color. I’m at a loss as to how to make the new color look the same on both sides. Is this possible with a plain garter stitch?
Frustrated! :??