I'm also doing a citron project - but with a laced type (Sebrights) - so I have had much the same questions!!
There are two aspects to a project like yours and mine.
1) Introducing the right gene to get the colour your want; and
2) Dealing with the result of combining the two bird types
In terms of the color, The general consensus is that Citron is a diluted version of gold. There are more than one set of genes identified that act as diluters in this way. On the kippenjungle calculator they are listed as Champagne Blonde and Dilute. My advice is to avoid using silver birds in your project - stick with gold. Perhaps someone with experience with the Spitz and Brabanters can add more here on how the dilute works on those birds
Then you're going to need to plan for the inevitable blending of all other factors. Bird size, comb type, henny tailed or not?, what ground colour, shank colour etc etc. Blending two types has the potential to break away from the traditional "standard" for each and you need to decide what it is that you're aiming for. To breed back to one particular standard or have something of a mix that you like for your own purposes.