Stain is not taken by capsule in gram negative organisms becasue the capsule consist of -

Slime has a little affinity for basic dyes and is not visible in Gram stained smears . Special capsule staining techniques are available usually employing copper salts as mordants . Capsules may be readily demonstrated by negative staining in wet films with India ink , when they are seen as clear halos around the bacteria aganist a black background Methods used for demonstration of Capsule: 1.Indian ink(Negative staining) 2.Serological Methods 3.special capsular staining Ref :Ananthanarayan & paniker's Textbook of microbiology 9th edition pg no 18,C.p.Baveja pg no 17