In order to place a colored hex piece to activate a terminal you need that same colored piece nearby to connect to. You can't just drop any color piece anywhere, it has to connect to its own color usually from a piece you had to clear earlier.
Also you can drop a station hex made from the specific color you want to link around where you place it. Is that what you are talking about?
It makes it more of a pain but you still can. Go to the guild building, and trade in 5 of the color hex you want to place for a station hex of that color.
Then you can place the station hex on any safe gray hex spot on the level. Once you have the colored station hex placed you can connect that same color hex piece to the station until you reach the terminal.
Need to link more color hexes of the same color and have the number match the second or past like 100+/100. More than 1 Terminal can be linked and the "shafts" between Layers can be linked to continue to the next area.