Colour change

How can i change the colour of a particular object in 3d.

You can use Autocolour Rules. you can find plenty in the topics here and in Manuals.

From where i will get autocolour rules.

ask your admin to set the autocolour rules for the project permanently.Autocolour rules are set of rules that will determine the color and translucency of the object. please see 4.1.1 Selecting Graphics Colours Automatically in the Design Reference Manual - General Commands.

the two files you will need are:
1. des-col
2. des-element-rules

--Settings
--Graphics..
from Graphics Settings click Colour
-- Auto Colour Rules...
  Element Rules...

How can i change the colour of a particular object in 3d.

By 'particular object' you may want to try the

add ce colour red

More info here and check also the macro : http://52.178.132.139/showthread.php?t=14954

Enhance ce col red (You can choose any col of your choice provided it is defined in PDMS)