Most e-readers are optimized to display monochrome text, with each pixel of a font placed by the operating system and software to maximize legibility.
On the other hand, a screenshot captures the pixels as shown on the display, which are modified to reduce aliasing and to make letters appear rounded. To do this, the letters are displayed in shade of gray, as shown below.
If you must include a screenshot of text in an e-book, there are a few things that could help.
- Increase contrast and reduce gamma with an image editor such as free IrfanView.
- Reduce the number of colors to two, i.e., black and white.
The image below shows both effects applied.
Far better, though, would be to take the screenshot and to convert the image to text using optical character recognition, OCR with a tools such as FreeOCR, as shown below.
With care, this will reproduce the text of the screenshot as characters, not as a gray-scale image.