I just want to know how the soldermask can be changed to coloured one and what will be the cost of PCB with coloured soldermask and silkscreen. Is there any other advantage of using colored pcb or only asthetics.

 

I just want to know how the soldermask can be changed to coloured one and what will be the cost of PCB with coloured soldermask and silkscreen.

For example, at PCBWay on that page you can select solder mask colour:

The top row are all the same price, $5 for 5 small (40mm x 40mm) boards.
The second row is a lot more expensive$38.43 for the same thing.

Is there any other advantage of using colored pcb or only asthetics.

I am not aware of any other advantage, but others may be.

 

I use JLCPCB.com