All the ebooks have color covers, and may have color illustrations - the eInk Kindle hand held units display ebooks in shades of gray.
A universal viewer/reader, Adobe Reader, is available free from Adobe.com for reading .pdf files and particularly ebooks. There are other .pdf readers available, some specially designed for particular devices. The beauty of ebooks in .pdf format is that they look on the reader exactly as the designer and author planned. Footnotes can be exactly on the page they refer to, pictures are placed where the designer wanted, and indexes can locate exactly. .pdf ebooks don't flow like .epub ebooks do and if you make the text larger the whole page grows and you might find it hard to navigate.