One of these calculators?
https://kippenjungle.nl/kruising.html
http://kippenjungle.nl/breeds/crossbreeds.html
The second one lets you choose breeds at the top, but it's missing some colors for some breeds. (I don't think it's got your marans or your salmon faverolle, for example.)
A basic explanation: for the hen and the rooster, there is a list of genes.
Each gene has an abbreviation, and those are in a list down the left side of the page: E, Co, Db, Pg, and so forth.
Then there's a drop-down box with choices for each of those genes. Some have just two or three choices, others have lots of choices.
Anything with + at the end is considered the wild-type: what the wild ancestors of chickens had. It's also the default when you open the calculator. When talking about what genes a breed has, it's common to not list the wild-type ones, just the ones that differ from that.
Sometimes I just start changing genes and look at the pictures--may not help me do a specific cross, but does teach me things about what the genes do.
Sometimes I google for the color genetics of a particular breed, but that only works sometimes.
For the specific breeds you named:
Marans--depends on what color.
Salmon Faverolles-- as best I can tell from google, people are still arguing about some of the details of that one! They agree on this much:
--E (top spot in the calculator): E^Wh/E^Wh Wheaten
--S (10th down from E): S/S Silver (male) or S/-- Silver (female)
I found someone saying it had Mh/Mh (Mahogany), and someone else saying it did not, and someone else saying it had that and another gene.... At that point, I gave up for now