Dahlias are octoploids which pretty much guarantees that each child will be different. However, with dominant and recessive genes there is a tendency for many open pollinated dahlias to revert back towards open centers, eight petals and a pallet of the colors one can mix up using red, white & yellow.

"Lets say a yellow flower is pollinated by a pink flower....Would it be safe to say that the resulting seeds will produce a pink and yellow flower that is in between the size of the 2 parent plants? What if pollen from more than one flower from different dahlias pollinates a single flower of another flower.... Will there be traits of 3 or more flower combined? "
A yellow flower pollinated by a pink flower will produce several color possibilities. Probably there will be a very few solid yellow, very few solid pink, many blends of the two and orange which in dahlias is a blend of yellow and pink pigments. There is possibility of solid red also. As the previous post said they are octoploid and the with extra genes, you get lots of possible color combinations. I would rule out getting any purple flowers and white would also be very rare. But all could happen.

The same is true for size. Many will be the same size as one of the parents. Some could be smaller than both parents and some could be larger than both parents. Most will fall into what you say about half way. It would be very unlikely that any flower would be two sizes bigger or two sizes smaller.

A dahlia flower when pollinated by a bee can get pollen from any flower the bee visited that day. If the bee visited all of the flowers in your garden that had pollen, the flower could have seeds pollinated by numerous different flowers. Having said that, the flower visited by the bee just before visiting your flower is the most likely candidate. Each seed can only have one pollen parent and by the way most dahlias are not self pollinated but some can be.
Most seedlings look terrible. about 2% look good and 1% look very good.

Apologies for reviving an old thread, but as an enthusiastic grower-from-seed, my view is that if you have a bit of extra space and time it is immensely rewarding to grow plants of many genera from your own saved seed. Yes, it's probably true that in the case of Dahlias you might end up with a bunch of undistinguished-looking plants, but equally you could end up with something lovely. There are few things more exciting than waiting for the first blooms on your seed-sown plants.

I've grown (and lost through frost) some attractive plants from saved seed of what was probably 'Bishop of Llandaff' crossed with an unknown red double-flowered Dahlia. Some of the offspring were rather gangly with floppy-stemmed single flowers. Others were more compact, with semi-double flowers. Flower colour ranged from orange, through reds and deep pinks. Leaf colour also varied but most were dark. The anticipation of their first blooms was like being a child waiting for Christmas!

Dahlias are so easy to grow from seed and such fast growers that it seems a waste not to give it a go.

On another thread, Rachel wrote this:

"Are all seedling dahlias pretty much the same way ? Messy open centers,
not uniform in bloom ect... I bought some cactus dahlia seeds from pase
or somewhere similar. I do believe I'm going to be picky with my dahlias
like u are! Im picky on which varieties I like w/named dahlias. If this
is the case I'll be giving all my dahlia seeds away!!"

I'm posting my answer here, as the information from past posters here might be helpful to her and to the new grower Sidhant in Dubai.

Rachel,
Lots of casual growers don't mind the floppy informal look of an
open-centered seedling, but many serious dahlia growers go through a LOT
to keep such 'junk' out of their dahlia beds. Conversely, hybridizers plant
thousands of seeds to find a few with the fully double form, and grow
the tubers of those rare plants for several years before naming and
distributing it as its own distinct variety.

As you have lots of room to grow, you might well enjoy planting the seeds six inches apart in a side area. It might be fun to see what you get. As space is limited for me, my inclination is to stick with named varieties that have pleasing colors/forms to my eye; however, lots of my growing buddies have gotten a LOT of enjoyment in trying their luck with a handful of seeds. As the London grower Ornata said, "The anticipation of their first blooms was like being a child waiting for Christmas!"

Hope this helps,
CiCi

'Junk' that is typical of seedlings, with 2-3 rows of petals
and a large pollen center. The bees sure don't mind, though! This
actually came from a regressed fully-double variety, with all the blooms
open-centered. I dug and composted the tubers, as I am a purist when it comes
to such things. Lots of casual growers might have treasured it, and that works
out best for the bees.

Late in the season, more and more dahlia plants will 'pop' their centers sooner, right before frost. This one is Dark Magic, which was just fine a month before.

Really good fully double varieties have little to NO
pollen center, making a great show flower, but not so good for the poor
frustrated bees. It also makes it hard to get seeds from such tight centers.