Yes the livery did exist .... briefly .... only a couple of locos in this class AFAIK (106 and 110) were repainted into this livery before the WP&YR was nationalised and the scheme changed to All Over Blue.
All the Alco / MLW locos (of the type modelled by LGB) were originally painted in the Green and Yellow livery, this was being changed to Blue and White around 1977 before the WP&YR was nationalised in 1980 and the livery was amended to All Over Blue. All the class had received the All Over Blue livery when the WP&YR ceased operations due to its main source of income closing down.
The original order for these Alco locos was for seven units (101 to 107) in 1969 but two were immediately destroyed in the 1969 Roundhouse blaze (102 and 105) along with the other WP&YR diesel modelled by LGB (Plymouth Shunter No.3). A further three locos were therefore ordered (108 to 110). The carcasses of the two fire casualty Alco's were kept as body parts donors until cut up in 1995. When the WP&YR closed down, the five oldest Alco's (101 / 103 / 104 / 106 / 107) were sold to a railway in Columbia but were bought back again when the new private WP&YR venture re-opened in 1988, now mainly for the tourist traffic from passing cruise liners. When the WP&YR started up operations again in 1988, the livery stayed All Over Blue for a while but the livery has recently reverted back to the classic Green & Yellow.
Ironically, freight (ore) business was at a high in the early 80's and the WP&YR had ordered four larger diesels from Bombardier (111 to 114) ... only 114 was delivered (in All Over Blue livery) before closure and so the other three were sold to another operator.
In a nutshell then
Green / Yellow livery ..... 1954 til 1980
Blue / White livery ........ 1977 til 1980 (only locos 93, 97, 98, 100, 106 and 110 received this livery)
All Over Blue livery ........ 1980 til 1982 then 1988 til as late as 2007
Green / Yellow livery ..... 1990 to present.
There was obviously some overlap dates with the liveries.