Plastic to stone?

I´d look for some pictures in colour of the type of stone wall you want. There are many different types of stone out there and they all look different.
You could look through books or even go out on town looking for old buildings which have the materials you´re trying to replicate. Take pictures and go from there.

try useing a sponge with the incorperated paint you're useing and a lil' dry brushing after all is said and done.

So basically just paint the plastic like a gray for example, take some of that very gray and lighten it or darken it ( that's up to you ) get an old sponge and get it wet so it absorbs the paint fairly well, then dab it in your lightened or darkened paint and just experiment on something first then see what you like the best. I would also dry brush it after you're done to get the spots you missed with the sponge.

hi , in the early years i've build several plastic wall for dio's and i did like this : paint the bricks in orange,oker(dirty yellow),brickred,dark red and the concrete with dark grey. then for the grouting(between the bricks) used dark grey mixed with washing-up liqued, cover the hole wall and wipe then the exess of with a piece of cloth. let it all dry and then drybrush the bricks and you can then use pastelcolours(chalk) for extra dusting. hope it will help you

Overall base color.
Then stipple with a toothbrush by fllicking it with your thumb. Use a light shade, a slightly darker than base and a really dark tone of your base color for this.
Let dry and cure.
Mix up a dark wash and apply.
Let dry and cure.
Dry brush with light color. Very light color.
In addition, towards the lower part of the wall, drybrush with your ground color. (dirt and mud and dust is always kiked up onto lower parts of walls) On upper edges of sills, bricks or stones, use a very light colored drybrushing . (Replicates, sun bleaching, light dust etc.)
Add things like bird droppings, rust stains from metal that may be attached or connected to the wall somehow. Things like pipes or wires. Fixtures, balconys, flag poles, lightnng rods etc. Can have streaks of rust under them. Rain or water staining form mold and mildew growth in the rock, brick or stone. Add some lichen or moss etc, grass that can grow from bird droppings, scatterings etc. Especially on old established buildings or ruins.

Mike

even just painting your wall with a base color, and dry brushing with 2 or more similar shades of the color, and then washing with an even darker clor has worked nicely for me in the past.