Hello and thank you for contacting us.
There are a few reasons that roses might change color. I'll go through them and maybe one will fit your situation better than the others.
- Weather can have a subtle affect on rose color. Cool spring weather usually produces more intense or "saturated" colors, while high heat tends to make colors less intense. A few of my roses have even burned on the edges in the high heat of the last week. Weather can make roses lighter or darker, but does not typically change colors and it doesn't affect just part of a bloom
- Growing conditions (humidity, irrigation, soil fertility, soil pH, and more) also influence rose color, however this typically affects an entire plant more or less equally.
- Grafted roses may send up shoots from the root stock that are a completely different rose. You can find an explanation at this link: https://www.gardeningknowhow.com/ornamental/flowers/roses/why-roses-change-color.htm
- Mutation or "sports" are changes to a small area of a plant. It could be different colored flowers on one stem, it could be different leaf shapes or colors on one branch, or a slightly different fruit. If these mutations are stable (don't revert back) and useful or pretty, then growers may propagate them and create a new plant variety. In some cases, those mutations are not stable, and a plant might revert back to the original form/color, or it might produce all sorts of new color/form combinations.
- Special varieties. You might have a special variety of rose that changes color as the flower ages. Some rose varieties are known for starting out as one color and changing as they mature. 'Jacob's Coat' is an example of this. If you look up pictures of this rose, you'll see blooms in yellow, red, and pink on the same plant, sometimes on the same branch.
I hope this answers your question. Please let us know if we can be of further help.
In my first answer, I incorrectly said Jacob's Coat, when I should have called it Joseph's Coat. Sorry about that!