There are no photos and we need your zone.

Monrovia shows 136 fast growing vines for zone 8: https://www.monrovia.com/plant-catalog/search/?type=14,15&cold_zone=8&landscape_use=5,23&growth_rate_id=1

Right? Take it easy tatts.

Photos up, and upright. :-)

OK, for zone 10, Monrovia only shows 89 choices, but you should be able to find something there: https://www.monrovia.com/plant-catalog/search/?query=&type=14%2C15&cold_zone=10&landscape_use=5%2C23

My lord, tatts. Just chill. If people posing questions on Houzz pisses you off so much, why do you bother participating?

Mandevilla will grow quickly and flower for months.

Lantana.

What does the reference to the plants being potted mean? Plants being planted out of pots and into the ground, or being kept in pots the whole time?

If you want to be sure to cover those walls in a single season, consider Morning Glories and Moon Flowers. These are annuals grown from seed. Suggest both because the Morning Glories close at night when the Moon Flowers open up. Less expensive, more temporary.

We're in SoCal. One suggestion for fast growth is cape honeysuckle. You could take your photos to a local nursery for suggestions. Great list of the 89 from grover!

https://www.monrovia.com/plant-catalog/plants/2151/orange-cape-honeysuckle/

Another idea is trailing nasturtiums. They are an annual most places and planted from seed (sometimes available already planted in 6 packs or flats). They are gorgeous and very fast growing.

Don't fertilize or you'll get more foliage than blossoms:

Plant nasturtium seeds in early spring in moist, well-drained soil in full sun. They can grow in partial shade, but they will not bloom as well. Nasturtiums prefer poorer soils and they do not need fertilizers (unless your soil is extremely poor). Fertile soil will produce fewer blooms and more foliage

Hi Embothrium "What does the reference to the plants being potted mean? Plants being planted out of pots and into the ground, or being kept in pots the whole time?"

Plants being kept in the pots the whole time. This hillside is full of fractured rock. As such planting directly into the rock/soil would be a major undertaking.

Thank you all for the great advice. I have my research work cut out for me!

Never heard of zone 10 in Northern California... I’m zone 9 and anything in a pot trailing on a hot wall in the afternoon would cook...

Planting annuals sounds like a great idea. I'd talk to someone at your good local indie nursery to find out what grows well in your area, since there can be some microclimates and you want to choose something that will look great in Sept. I like Nasturtiums but for some reason I thought they don't like heat (so if you are zone 10 the July-Aug heat might get to them.)

If you have no dogs, I'd plant Morning Glories to cover the fence. I've grown them in zone 9 and they are very fast climbers. They are aggressive growers and will climb trees, walls- whatever is near their path. I kept them in a pot - which I placed on top of dirt and they still grow 2 stories tall in a summer.. When their vines touch dirt, they grow roots at that junction. I had them at my condo and they were growing up,& over the slow-growing star jasmine & up the stucco to the 2nd floor. The good news is they were easy to remove/ rip out.

Otherwise, I'd plant small ornamental trees that flowers all summer CRAPE MYRTLE. The lovely flowering trees will steal the focus away from the fence,

Also, AGAPANTHUS for full sunny spots between the trees & halfway between the retaining wall and fence..

Nastutiums for 1 or 2 feet above the retaining wall. Nasturtims are edible by humans and dogs... Use them in your salad, taste like raddichio lettuce. I've grown trailing vine nasturtiums- they never climbed/grew upwards or over rocks-, only along the ground. Like other vines, they will root wherever they touch dirt..

You want this to look good in September? Nastutims seem like the best shot but will they look good that late depends on exposure [it looks very sunny there] Mine start to fade and aphids cover them in the fall. Unpleasant. You will need a more permanent plan long term.

Oh and morning glories are regarded as kind of a nuisance plant/weed in this zone[9}

I've never heard Morning Glories or Moonflowers to be a weed, but they will climb anything in their path, which is why I suggested using them on your back wood fence. They attract a pest small white flies.... On the back fence - they will be away from your guests. So, no one will be bothered,

You might be tempted by bright and beautiful flowers of BOUGAINVILLA, but it has big thorns, when it's mature. It is another aggressive grower.

Oleanders have beautiful color, but are another toxic plant. I've seen them growing wild in SoCal.

What about Dichondra silver falls., especially with other plantings

Thunbergia

FYI- I had silver falls Dichondra growing in a planter.... It has thorns and the sap causes a burning rash.

One easy way to add color or lighting to the backyard- Shepherd Hooks

Stake these in the ground just above the wall and suspend a "color bowl" aka mixed seasonal flowers and/or lights. It will take the focus away from a plain wall & no need to wait for things to grow. Easy to water with the a watering can.

Here- they used vases

I would power wash the fence and beef up the grass. The wall is not offensive. A row of potted plants can do wonders. Baby them
in a semi protected area with a bit of shade
until the wedding.
Ask a nursery about Scoevola in your area.
It is especially hardy once established. Blue or purple and sometimes white. Looks great with yellow or hot pink flowers.
I also question your comment about zone 10....I
am in SW Florida for sure a zone 10....It’s a jungle out there!!!!!!!!!!!!
Is the wedding party at your home?

PS. Bougainvillea takes a long time to become established until it is ‘lush’.

Rather than wait for vines to cover .....use wire to make a design