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Iris do not change color, but growing conditions can make colors more or less vibrant.
Over spray of a herbicide have been reported to change colors temporarily.
Iris can actually drop seed and sprout new flowers that may not be true to the parent flower due to bee pollination from other iris color.
It is reported that their are a few varieties that will 'sport' and new color.
The variety Honorabile has a history of this rare event.
It is also possible that a garden area had previous rhizomes and you had original yellow Iris that had never bloomed until they were lifted and moved.

I would certainly go ahead and trim them back to about 4 inches high. This will clean up the garden bed and still leave enough greens.

https://www.gardeningknowhow.com/ornamental/flowers/iris/growing-siberian-iris.htm

There is really nothing that you can do to reduce the height of your Iris stalks. You can stake them if they tend to fall over.
Hybridizers do try to produce plants that grow a sturdy stalk. There are also many cultivators that are smaller or even dwarf varieties.

You can dead head the flowers after they are spent and cut down the flower stalks after they have completed blooming.

https://www.gardeningknowhow.com/ornamental/bulbs/iris/iris-plant-care.htm
https://www.gardeningknowhow.com/ornamental/bulbs/iris/divide-bearded-irises.htm

Cover the rhizomes with 2 to 3 inches of soil and space them about 2 feet apart.
This allows enough space for the iris to spread.

I would plant your Daffodils near the Iris but don't plant them directly next to each other.
You will need to lift and divide both the Iris and Daffodils at times and if they are planted separately, this task will be much easier.

https://www.gardeningknowhow.com/ornamental/bulbs/iris/growing-siberian-iris.htm
https://www.gardeningknowhow.com/ornamental/bulbs/daffodil/daffodil-planting-care-tips-how-to-plant-daffodils-in-your-garden.htm

Diane, after giving Siberian iris a year or two to settle in, if there are no flowers it's usually because the plants aren't getting enough sunlight. They need at least 6 hours of sun and are happiest with full sun.
If that's not the issue, are they planted in alkaline soil? They prefer neutral to slightly acid pH.

There can be different reasons for color changes in flowers, environmental changes and even accidental herbicide drift can cause the flowers colors to change.
Here are some articles with more information.

https://www.gardeningknowhow.com/ornamental/bulbs/iris/why-an-iris-changes-color.htm
https://www.gardeningknowhow.com/plant-problems/environmental/fixing-color-faded-flowers.htm
https://www.gardeningknowhow.com/ornamental/flowers/clivia/clivia-color-change.htm
https://www.gardeningknowhow.com/ornamental/shrubs/hydrangea/change-hydrangea-color.htm