Does changing the size and color of text help in SEO? I see a lot of sites doing it, such as randomlinks.com/
since you can define both color and size of thext inside your CSS file, and your CSS file is NOT scanned by a search engine, the answer is no. Search engines are color and size blind. However if you put <font> tags all over the place, that will hurt SEO and it's not really valid HTML any more, so stick with all sizes and colors inside your CSS and you should be ok.
I don't know about Search Engines not reading CSS because Google has fetched my CSS file on a couple of occasions. don't use randomlinks as an example for anything, that page is a perfect example of what not to do. you will get very little benefit from a link on that site. Use header tags, <h1> ect. bold, italics, lists and underline to highlight key phrases in the text on your page.
is it because that site has a lot of links that makes it unhelpful or is it because of the font tags?
it is because the site is obviously selling links and Google doesn't like that. plus the site has many outgoing links and the site has no clear topic. Its just a page full of links that will not benefit you in any way. Who visits that page? If you were surfing the net and came across that site, what would you do? I don't know about you but I would leave immeditely. if you buy a link on that page do you think you are actually going to get traffic from it?
It looks like "RandomLinks.com" has a PR of 3. So wouldn't the people who puchase the links at least get the benefit of some of that PR? (sure, divided by the 324 links available, but some PR nonetheless).