I am not going to go into this in detail but it is possible to just paint the inside (and chimney breast) of your builder’s opening – whatever it is made of (brick/stone/slate etc.).
Painting a fireplace can yield quite amazing results and for very little effort.
The pictures shown are examples of unused fireplaces (whilst I search my hard drive for one of my own painted breakouts). Note that brick, stone, slate all look good painted.
Note that it is very hard to get paint off again so you have to be sure!
Get rid of any loose mortar etc.
Fill any holes with mortar (PVA first)
Apply first coat of paint (first coat will likely look awful as it will pick up any soot/dust etc.)
Allow to dry. Apply second coat.
Add more coats as required.
I’ll leave you to ensure you buy suitable paint for the surface.