Two questions ... (1) Using Humbrol enamels, what would be a suitable colour match for the Leopard 1A2? If I remember correctly, the colour is referred to as 'gelboliv'. Italeri suggest Olive Drab, which would be either 66 or 155.
(2) Other than the 4-colour splinter camouflage scheme, are there any other options for the Strv103C? I had in mind an overall green with a winter white disruptive camouflage pattern, but wasn't sure which green to use.

Don't use olive drab for the Leopard. Gelboliv is a distinctive color the German Army used.

http://theminiaturespage.com/boards/msg.mv?id=199198

I use the Model Master enamel gelboliv.

For Early German colour I use a mixture of

4 parts Tamiya XF-51 Khaki Drab
2 parts Tamiya XF-74 JGSDF Olive Drab
1 part Tamiya XF-57 Buff

Picked this up a while ago from some fellow Modellers!

Hi Paul
As far as the Leo is concerned, Revell Enamel RL32142 Yellowish Olive or at a push acrylic RL36142 should see you right.