In an expression, you set the R, G, and B like this:

lambert1.colorR = 0;
lambert1.colorG = 1;
lambert1.colorB = 0;

You can connect another float3 attribute to lambert1.color, which is a compound attribute that includes colorR, colorG, and colorB.

Look in the Node and Attribute Reference to see this stuff -- these attributes are listed on the page for the Lambert node.

-- Mark