Why do the colors look slightly different after converting an image?

Cause one: the color profile is not carried over

Some images carry a description of the color space they were saved in, called a color profile. Recent phones sometimes save in a wider space than usual, and design software may export in a space meant for print. The program showing the image reads that description to display the colors as intended.

The Image format converter draws the image onto a canvas inside your browser and saves it again, without copying the color profile into the new file. The browser converts colors as it draws, but very saturated colors that do not fit the standard space get pulled to the nearest one it can show. For ordinary product photos the difference is small and most people never notice it; deep reds, vivid greens and punchy oranges in photos from recent phones may look slightly calmer.

Cause two: transparent edges in JPEG

When you convert a logo or a cut-out to JPEG, it is painted over the background color you choose. Semi-transparent pixels along the edges blend with that color. If you chose white and then place the image on a grey page, you get a light fringe around the edges that looks like a color change. The fix is to pick a background color that matches where the image will be shown, or to choose PNG or WebP so the transparency stays.

Cause three: low quality

In JPEG and WebP, the lower the quality, the more detail is dropped, and that includes color detail. Saturated colors that touch, such as red text on a blue background, can bleed into each other slightly. At the default of 90 this rarely shows; at 60 or below it becomes more visible.

How to keep the shift small

1. If the image comes from design software, export it from there in sRGB before converting. What you see before and after will then match closely.
2. Compare the preview with the original on the same screen, not on two devices, since screens themselves differ in how they show color.
3. For transparent images going to JPEG, choose a background that matches the page they will appear on.
4. Keep quality at 90 or above for images where color matters, such as fabrics and cosmetics.

If exact color is a hard requirement, for print or for a catalogue that must match a real fabric swatch, do the conversion from the source file in design software that manages color, and use the tool for everyday web and social images.