When we list equations it's Father+Mother=Child
Human+Neanderthal means a man from Earth and a woman from Nean
Neanderthal+Human means a man from Nean and a woman from Earth

These Neanderthals like space travel for the pussy it brings. They like experimenting sexually with different species. But they still prefer to marry Neanderthal women. And since no one else in the universe is lining up to have sex with Neanderthal women, the women insist on marriage before sex. Who their husband sleeps around with besides her is his business. She's the one that gets the inheritance.

Human

The effects of Neanderthals breeding with humans is seen all around us. However, we have diluted DNA. When it's fresh, who's to say what happens?

Neanderthal men don't see much difference between Neanderthal women and Human women, so it doesn't have the same thrill for them. Neanderthal women think that they might be able to sleep with a Human man, so that is more interesting. However Human men often have enough Neanderthal DNA that they don't see Neanderthal women as much different from Human women. If there are no Human women around, a Human man might be willing to sleep with a Neanderthal woman, but then again, the idea of marrying one just for the chance at a bit of sex isn't appealing to a globe-trotting astronaut.

Glace

As for a Glacian breeding with a Neanderthal, any hopes for a boy are dashed (100% girl guaranteed). The male chromosome is not strong enough, but many of the facial features (like noses and mouths) are. Thankfully the Glacian foreheads and eyes are the stronger DNA. Skin tones don't change because they have similar skin tones to begin with.

Neanderthal+Glacian=Girl with Glacier blue hair. Meat is the staple of her diet. Able to digest a few cooked vegetables. Still needs lots of minerals in her diet (saliva can dissolve rocks).
Neanderthal+the next generation=Girl with light blue hair with dark blue streaks. Able to digest all cooked plants. Still needs regular mineral supply in her diet. Neanderthals have created mineral licks (similar to salt licks).
Neanderthal+the third generation=Girl with dark blue hair with light blue streaks. Able to digest all cooked plants and some raw plants. Might have the habit of licking stones.
Neanderthal+the fourth generation=Girl with dark blue hair with black streaks. Now that the Glacian hair is almost gone, the other chromosomes have a chance of changing. Still loves meat but can digest all plants whether cooked or raw and gets minerals from salt.
Neanderthal+the fifth generation=3 girls for every boy with black hair. Normal Neanderthal diet.

Darsite

For the Darsites, the eyes are the important feature for their survival. It's the dominant gene. The next generation will have the bottom liner marking and have a greater chance of having a pale eye colour.

Neanderthal+Darsite=either gender with black lips, blue marks under the eyes and pale eye colour. Hair will be darker.
Neanderthal+the next generation=black lips, no eye marks, pale eye colour. Hair will be black. now that the Darsite eyes are gone, the other chromosomes have a chance of changing.
Neanderthal+the third generation=dark red lips, darker eye colour, black hair, broad shoulders, shorter height
Neanderthal+the fourth generation=red lips, brown eye colour, black hair, slightly prominent brow, broad shoulders, stocky stature

Fontian

For Fontians, the hair colour is the dominant gene. Their platinum blond hair mixes with the Neanderthal dark hair to create grey haired babies.

Neanderthal+Fontian=grey hair, Neanderthal nose
Neanderthal+the next generation = light black hair, slightly prominent brows
Neanderthal+the third generation=black hair, shorter height, prominent brows

The next generation+Neanderthal=light black hair,
The third generation+Neanderthal=black hair, shorter height. It is in this generation that the brow begins to become prominent because of the continual breeding with Neanderthal Females whose brows are less prominent than male brows.