Ive never put filling in a cake what is good to use say for a yellow or marble.

buttercream icing; fruit jams; cream cheese mixed w/some non-dairy whipped topping; pudding; pie filling; the list is endless
Match the filling flavor to the cake flavor.
Marble - I'm assuming is choco & yellow or white cake: choco buttercream; peanut butter mixed w/cr cheese; raspberry jam; even strawberry or blackberry jam. What are your likes?

I was curious about a filling for a yellow cake as well.

i love raspberry filling in butter cakes... you can use raspberry jam.. buy the sleeve of it from countrykitchensa.com... globalsugarart, too. i believe.. i torte.. make dam.. put in some buttercream that i am using.. then the raspberry filling.. you don,t have to use the buttercream with the filling.. i don,t always.. you can make your own filllings from any fruits.. there are lots of filling recipes on this site.. hth