There is a photograph of the second prototype Saro P.531 G-APNV at Farnborough Sept 1958 at the following link:-
Westland Scout - G-APNV - AirTeamImages.com.
In Flight Magazine's report on Farnborough 1958 below it states that both of the first two prototype P.531s (G-APNU & G-APNV) were at Farnborough that year and both were finished in blue and white. I think when the aircraft first flew they were in the primer colours.
armstrong siddeley | hawker hunter | 1958 | 1- - 0394 | Flight Archive
I have a copy of Flying Review Oct 1958 which has a report on Farnborough 1958 and also includes a small black and white photograph of a P.531 flying at Farnborough and the colours whould appear to be the reverse of those shown in the AirTeam photograph. It would indicate that G-APNU's colours were the reverse of G-APNV.