Why did my seeds germinate green than leaves turned brown?

Because you somehow killed them.

Without a little more info and maybe some photos, it will be hard to determine why.

Yep, when green leaves turns brown, something went wrong. To give you a better answer we would have to know what you planted, how it was planted, what kind of planting medium, light conditions, heat conditions, watering schedule, etc.
Photos are always helpful but I'm guessing if your plants already turned brown you probably don't have pictures to share. Better luck next time...

Even if you don't have photos... if you can tell us what sort of flowers you planted, and describe what you planted them in and what else you did, we might be able to give you some help for next time

I doubt there are very many people here who haven't managed to kill some plants off... that's why we have compost piles

A few thoughts here ref seeds or indeed plants being potted on,
Lets have a look at a few basic things ref compost we should know,

Seed compost,
Now before you sow seed or plant anything on (re-pot) you should understand what all growing plants need and its simple because they are much the same as you and i as humans need
(1) light (2) warmth (3) air (4) and water,
Now you'll notice ive said nothing ref food And thats because seeds have a food supply in the seed cover
Enough for the seed to get started,

So now we need to look at (Light and warmth) Seed compost is very light and this helps the seed once its opened it's case (pod) to push up to the natural light and air,
also seed compost will provide enough heat to aid the seeds,
But seed compost has no food in it or any of the minerials the new to life plants will need to continue to grow into strong plants,

"Water" again these very young plants will need water "but" warm rather than very cold water and only a very light watering, (I only use a mist spray)
If you use to much water you'll drown the young seeds .

As soon as the seeds have germinated into young shoots (plants) you must keep these shoots in a constant warm situation, and remember your seed compost has No Food to feed the these young shoots and so you now have to have the correct potting on compost As these young plants will need re-potting very soon,
Again you need to know what your young plants need ie are they Lime lovers ??? Or acid loving plants
If you pot-on in the incorrect compost you'll kill the young plants. So only use the correct compost.

Over watering when you've potted -on is a favourit killer
, Also what sort of pot you choose,
Ie clay pot are a good heat retainer and also they do soak the water into the clay material and so you've a better chance of the water drying away unlike plastic pots that tend to just let any over watering water just sit in the bottom /saucer and this causes the young plants to rot.

Note,
If you look at the top of most pots you'll see a rim, this rim is the mark for you to fill your pot with compost up, do not go any further than that bottom of the rim, whats left of this rim is the amount of water that size pot is designed to take, and if you use this rim marking with the pot fully filled with compost up tp the bottom of the rim "you'll not over water the pot.
TIP?
I use whats called bottom heat for all my seeds, this is a large tray with edges approx 2inch deep and the floor of this tray has warming cables layed in a snake shape fashion and then the tray is filled (2inch) with light sand or you can use slate or grit, these fillings retain a nice warm area to keep the clay pots warm all through the evening lower temperture's and the plants roots grow better/stronger
I know this is a long reply "but all good stuff