Walls refuse to go transparent


I have made lots of walls for a building. I want to change the colour of the walls so that they have a lower opacity so one can see through the walls. However, when i click command + A and select all the walls and apply a low opacity colour with the paint bucket the walls remain opaque.
Any advice much appreciated! Thank you.
PS have attached an image after of applying the transparent colour.

Also paints the back faces

Make sure you are in Shaded with Textures mode. If you are in Monochrome mode you cannot see transparency. There is a toolbar you can use or go to View> Face Style

Your image shows that almost all your walls have no thickness.

If you are trying to make double sided walls transparent, you need to apply transparency to both outward looking faces - one on the front face of the outside, one on the back face of the inside. [That is, if you want them to be transparent looking in, but solid looking out?]

Here’s an example: from one viewpoint

Then from another:

And here’s the model - of a theatre set in the Studio at the Abbey Theatre. St Albans.
Studio v5.skp (649.4 KB)
And to do that, you need to get front faces outside on both sides of the walls with thickness.

Can you upload the .skp file and not just an image?