Load your Inkling up in SFM on a map.
Right-click on their model name under Animation Set Editor.
Choose "Add Override Materials"

Right-click on their model name again.
Choose "Show in Element Viewer" and "Model"

Choose "Materials" at the bottom of the list that comes up.
Find "Inkling_boy_teamcolor" or "inkling_girl_teamcolor" and click on it.
Over to the right, click on the color bar and choose a color from the window that pops up.