Sometimes the text might be deliberately grey, but the usual reason for this is anti-aliasing (using greyscales to smooth the edges of glyphs). This is often worse for serif fonts than san serif, and heavier fonts are usually better too.
If you are running (or creating) ePubs through Calibre try switching the font to san-serif (via --extra-css):
Code:
body {
font-family: sans-serif;
}
You can also add your own fonts for ePub, providing is it DRM-free and so modifiable via Calibre, see
Fonts and Epub - What works on Sony, Works for Pocket Pro too!
EDIT: see
New Firmware Upgrade coming Friday-Monday for a short discussion of anti-aliasing.