benji112

1 - open the vehicles.img in Spark, then edit the car you want (here I use police.wtd)

 
2 - select the livery you want (here it is bflo_sign_2) then export it

 
3 - select the livery you want to replace (here it is bflo_sign_1) then import the previous file you've exported

 
4 - save and close, then save and rebuild, then save, and exit; take your time, CPU is working a lot !

 
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edit : you don't need to put "+livery" in vehicles.ide if you're using only one livery, but it will use the first one, so it's why you have to replace the first texture by the one you want.

No, but if you want only one of those four liveries, just replace the 1st (something like police_sign_1, but name may change following the model) livery' texture by the one you want, in Spark, clicking "edit" while selecting the pstockade.wtd

Be sure to do it correcly (look at tutorials section of the website) and don't forget to rebuild after saving on vehicles.img window.