Color sharing banners

Basically, you have higher chances of pulling a red *5 unit (if I recall correctly) in this banner.
Honestly, my disappointment is immeasurable. I really wanted Even Speed Wave for my Lucy but I'm not gonna risk it.
If you're gonna pull, keep in mind that on top of Ares you still have the chance of an occasional pity breaker. I'm pretty sure there is an average of one 5* unit every 60 or so orbs (which might be higher chance for reds in this case)
I would pull if you don't mind building an Ares, but if you're just going for Karla, too many things can go wrong IMO.
Of course, this is just my opinion.

You'll have a better chance of getting a 5* since two focus units share a color but just be ready to accept the possibility that you might get more copies of Ares over Karla

Since the banner only has 3 focus units, this will be one of your better chances of pulling a Karla. By the math, sharing the color has a minuscule effect on your chance of getting Karla, and that's only when you build up a significant pity rate. If there were 4 focus units, all with their own color, you'd have a significantly lower chance of getting her, for example.

Basically for each orb the game rolls to determine if there will be a 5 star drop - if yes, then it selects one of the 3 units to drop. So in this case, it is a 66% chance to be a red orb with either Ares or Karla, but that doesn't diminish the fact that Karla still has a 33% chance to be the pulled unit. Hope that makes sense.

You're probably thinking of something like this
https://fireemblem.gamepress.gg/q-a/against-colour-sharing-myth

Generally speaking, red is an expensive colour for finding a focus 5* because there are so many reds that we just see more red stones (per 100 stones counting all colours, for example). I ran some numbers, and I could be off by a few orbs because I haven't updated the pools to reflect the introduction of YT units but the numbers are correct relative to each other.
If you want 2/3 reds on a legendary banner, the expected number of orbs to obtain either of them: 104
If you want Karla on a 3%/3% banner with three focus units:
and she doesn't share a colour: 143 orbs
and she shares a colour: 148 orbs
If you want Karla on a 3%/3% banner with four focus units:
and she doesn't share a colour: 190 orbs
and she shares a colour: 194 orbs

Note that all numbers given above assume no such thing as pity rate. Just estimating by mathematical intuition, I would expect pity rate to change the numbers by a number of orbs less than 10. It also means that if you walk into summoning using the numbers I provided, you don't care how many pitybreakers show up on your way to finding Karla; you expect her within the same number of orbs.

As usual, I do well to make a disclamier that RNG doesn't always do what mathematical probability offers for an expectation. Any individual summoning session can go better or worse.

For a 3-unit banner like this one, each individual summoning stone has a 1% chance of being a specific focus unit (3% focus divided over 3 units). So when you start the summoning session, there's about a 5% chance the session has a Karla in it. This is true independent of any color considerations.

Your goal when sniping units is to spend as few orbs as possible to determine whether that session has a Karla or not. You do this by only summoning on red stones, since nonred stones obviously do not have Karla. Shared focus increases the amount you spend in two ways:

1) There are more "bad" red stones, because 1% of orbs contain Ares, which would show up as red. If it had been Brave Lyn as the third unit, these stones would be colorless and you wouldn't have to summon on them. However, this has a rather small effect, as the vast majority of "bad" red stones are 3* and 4* units.

2) The shared focus resets your pity rate more often. This is a totally negligible effect, as pity increases very slowly.

That first effect is also why sniping green is so much better than sniping red. Green is a much smaller fraction of the 3* and 4* pools, so fewer "bad" stones are generated, and so you spend fewer orbs determining whether the session has the unit you want.

I was after 2 Karla, before I pulled her or Ares I pulled Lucina, Ike, Morgan (M). Only then did I pull an Ares, another Morgan (M) and finally 2 Karla.

This banner has great red odds but because of the split focus it's a bit of a crap shoot. It's a bit like aiming for one unit on a legendary banner.