At the moment, I have a custom skintone (some eyes, too) that's 1.0. So if a Sim with the custom skintone and a Sim with a geneticized skintone have a kid, the custom one will always override it, right?

So if I wanted to set it so the children will inherit the custom one only half of the time, how would I do that? Setting it to .50 would just make it fall somewhere between S3 and S4. The custom skintone is completely outside the normal range, and I don't want it popping up on random babies.

Does this belong in help? If it does, sorry.

the values for the "default" skin tones are .1, .3, .6, .9.
an unchanged custom skin would have 0.00 as its value; skins with that value would always be inherited with other skins.

a custom skin being inherited only half of the time; inherited skins would depend on the couple/couples.

edit:: if both couples have just the "default" skins, there would be 10 different skin combinations.
possible inherited skins (couple's skins on left, inherited skins on right)::
s1 & s1; skins that have a .1 value.
s1 & s2; skins of value .1 through .3.
s1 & s3; skins of value .1 through .6.
s1 & s4; skins of value .1 through .9.
s2 & s2; skins that have a .3 value
s2 & s3; skins of value .3 through .6.
s2 & s4; skins of value .3 through .9.
s3 & s3; skins that have a .6 value
s3 & s4; skins of value .6 through .9.
s4 & s4; skins that have a .9 value

If you geneticize skins, sims can get any skin value between the range of the parents, so if one parent has a skin with value 0.15 and the other parent 0.62, their kid can end up with one of those two skins or any skin inbetween them.

Ideally for a 50/50 chance between two skins, you'd want the skins to be in very close range for the game to pick only between those two, so something like 0.15 and 0.16, or 0.31 and 0.32, and none with the same values or with values in between.

Here is the definitive guide to all things genetical: https://rikkulidea.livejournal.com/21779.html It includes how to change/set everything necessary in SimPE, for all applicable things. (Skins, hair, eyes, etc.)

I'm sorry, I'm just even more confused now. I'm not very smart at some things.

There's no real way to set it 50:50 (except maybe using 2 custom skintones where you will get one or the other 50% of the time). The proceedure for skintones depends on the genetic value of each of the parents' skintones and what you have in your downloads folder (alongside the defaults). The value the game picks is always either equal to one of the parents or a value in between.

So say you have two parents, one with a value skintone of 0.1 and the other with a skintone value 0.4 - the kids will either get a skintone with value 0.1 (and you may have more than one in your downloads folder) or value 0.4 or value 0.2, 0.3 and, in fact, if you have skintones with values like 0.25 - any value in between the parents' values up to and including them. The more skintones you have, the more likely you are to see variety.

The actual numerical value of the skintone has no effect on it except to determine where on a line of values it lies. So reducing your skintone from value 1.0 to value 0.5 simply shifts it along the line. It wouldn't be available to the baby of parents with skintones of value 0.1 and 0.4 but would be available to a baby with parents with skintones of value 0.1 and 0.7.