I use the ORBx EGPB Sumburgh airport with PlayHorizon VFR Photographic Scenery Volume 5 Shetland Isles & Orkneys, but I have a problem. I like to fly in early Spring, but this results in snow on the ground at EGPB Sumburgh. Does anyone know how I could get P3D v3.3.5 to display normal spring textures rather than snow?

If you look in Orbx dir for that airport you'll see the textures used for that airport landscape under Orbx\FTX_EU\FTX_AA_EGPB\texture\ORBX_EGPB_VolGrass_SU. dds, which is summer for example.  Many of the other airports actually have different land textures for fall, summer and winter which would make this easy, just copy the texture to the name of the season. This one uses Volumetric grass, but I think it will also be easy, however I should stress this is a guess and some crazy stuff I would try.   There's a file ORBX_EGPB_PLC_VolGrass_ObjectFlow.xml in this airports scenery directory.  In it you can see the seasons and a guid that I believe represents the texture it will use.  I think below represents Summer which has the green grass.  You'd just need to replace this guid with the similar section for Spring which I believe is the one labled MONTH_OF_YEAR GREATER_OR_EQUAL 1 AND MONTH_OF_YEAR LESS_OR_EQUAL 4.  There's also another xml file that looks like the primary grass file actually, ORBX_EGPB_PLC_Grass_ObjectFlow.xml a similar change will most definately be necessary as well.  My guess is that this file is used if you have volumetric grass disabled in the airport config and VolGrass is just an enhancement for 3D grass.

Good luck Mr. Phelps!  We will deny having ever communicated this assignment should your sim be compromised.  Strange things happen when you tinker with Orbx sometimes. 

<DynPlacement.Document>
    <Object>
        <AltAGL>0</AltAGL>
        <GUID>{fd1fb22a-ded2-4bae-a625-28f62ee4d003}</GUID>
        <Condition>MONTH_OF_YEAR GREATER_THAN 4 AND MONTH_OF_YEAR LESS_THAN 9</Condition>
        <VisRange>6000</VisRange>
        <Lat>59.877690528</Lat>
        <Lon>-1.288860937</Lon>
    </Object>

Unfortunately, I have been unable to get this to work

I asked on the ORBx forums, and I have been told that there is no easy way of doing this.