I get versions of this question often. You have something in the garden, but this year it looks a bit different than it did before. There are a bunch of different things that could have happened to cause this change, and I’ll attempt here to make a complete list of them.
Trees and shrubs, including things like tree peonies and roses, are often grafted, so the part with the pretty flowers or delicious fruit is stuck onto the roots of a different variety, often not as pretty/useful/tasty but more vigorous and/or easier to propagate. Sometimes shoots come up from that rootstock and take over the plant. The best sign will be to see if the shoots that look different are coming up from the very base of the plant. Cut all the shoots with the off-type flowers or fruits as completely as possible to allow the desirable parts of the plant to continue on.
This is most common with variegated plants that are chimeras, though there are a few chimeral varieties which have multicolored flowers rather than multicolored leaves. I wrote about what a chimera is here, but the short story is: Many variegated plants are made up of two different cell types living together. If one cell type starts taking over, you loose the bi-colored effect of the variegation. As soon as you notice them, cut out the reverted shoots to keep them from out growing the multicolored parts.
Sports are chance mutations, and sometimes a flower will just up and change color. Usually, a sport will only change one aspect of a plant, most often color, while size and shape and everything else stays the same. New variegated varieties almost always come into being as sports. Sports will almost always be isolated to just once branch or section of the plant while the rest of the plant maintains the original color. If you like the sport, you might want to try taking cuttings. If it is an attractive sport of a popular plant, it might even be valuable.
Sometimes it isn’t that the original plant changed, but rather that it had some babies, and the babies look different. Confirmed self-sowers like phlox and aquilegia are notorious for this. You get a nice variety, but in a couple years, it has died out and replaced by a few seedlings which usually aren’t as nice. To prevent this, dead head after flowers fade to prevent seeds from developing.
Hydrangeas famously will change flower color depending on the pH of the soil in which they are grown. However, this is rather an anomaly, and soil conditions have no impact on the look of flowers for the vast majority of plants.
Some flowers will change their look, sometimes radically, depending on climatic conditions, often getting darker or lighter depending on temperature and the amount of sunshine. So if a plant looks different in a year when it has been unusually hot or cold or cloudy or sunny, it will probably go back to the normal coloration once your weather returns to normal.
Often plants you get at the nursery will look quite different once they’ve spent some time in your garden. Many plants from the nursery will have been treated with chemicals called plant growth regulators which are used to keep the plants short and compact. As those compounds wear off, the plants will grow taller and looser. Also, commercial greenhouses are usually warmer than your garden and often exclude UV light which can chance the coloration of a lot of flowers.
If your neighbor gave you a clump of their favorite old bearded iris, but after a couple years the iris they gave you is replaced by a different color or a different plant altogether, they likely accidentally included a bit a different, more vigorous plant along with the one they gave you. Your best option here is to lift up the whole clump, wash all the soil off the roots so you can see exactly where one plant ends and the other starts, and just replant the ones that are true to type.
If you are spraying something like glyphosate (active ingredient in roundup) sometimes a bit of it can drift onto plants, not enough to kill them, but enough to cause them to be deformed or discolored. Glyphosate drift in particular will often cause flowers to be bleached out, white or nearly white. In this case, just be more careful next year, and everything should return to normal.
Okay, let’s just admit it. We don’t always remember that we moved something or what we bought or exactly what a new plant looked like last year. Sometimes there is no great scientific mystery beyond the fact that we’re all busy and don’t keep as good records as we know we should.