The rocks are already

...sounds like it's a moot point now...

Talus Painting

I paint the plaster debris in a separate container and set it out to dry before applying it as talus.

No preference

I've done both and I don't really have a preference - I'd say just whatever is most convenient.

I would personally paint

the rocks first before gluing them in place.  You never know what effect the glue will have as far as absorbing paint the same.  It sounds like Dave got lucky with his.  I'm never that lucky.

Thank you all for the reply.

Thank you all for the reply. I think I will paint those in some container first and then glue it on.

Jure

Definitely Before

Jure- Michael was right- paint them before you glue them or the glue won't let the paint soak into the plaster and it won't match your rocks that were painted without glue on them. Ask me how I know!

Craig

Colors

I'd paint them before, too, but use a slightly lighter color than your final one.  Then, once the talus is in place and the glue has dried, use an alcohol and india ink wash (or your favorite weathering wash) to darken to the final color and highlight the crevasses and gaps created when you placed the talus.