Hi Ricci,
Yes, if you want to place text on the floor of a 3D Floorplan, you should indeed use 3D Text (as sceen text is always stuck to the screen itself).
When creating the 3D Text, in the dialog, uncheck "Extruded" and it wion't be 3D just faces (grouped) you can paint as you like.
Now open the group, under View > Face style change to wireframe mode, with a selection box, select all the edges and hide them. This will prevent the text looking awful from a certain distance (edges are always displayed at 1 pixel - even if the face they surround is already smaller on the screen itself). When done, change your face style back to shaded with textures or whatever you were previously working in.
I hope this would solve your edges colouring need, too, but you can still do it under File > Styles > In model (small house) > Edit > Edge settings - at the bottom, set it "By material".