Possible? Yes. Although it is possible to do using CSS:
a[href="/wiki/User:LepszyMarek"] {
/* default text */
color: red;
/* multicolored text below this line (if supported) */
background: -webkit-linear-gradient(left, red 33%, lime 33%, lime 67%, blue 67%);
-webkit-background-clip: text;
-webkit-text-fill-color: transparent;
}
When it comes to browser support, the support is poorer than the national debt of the US. Since links can have a custom text, using a background image- created using an image editor- is no good either, since it cannot predict what the link text would be.
If you really want to have something that is supported more properly, JavaScript could be used instead, but that would require checking the page when it loads and also when its content is modified (e.g. when pressing Preview, or while loading more comments in user blogs), and i'd say that this is pretty ridiculous, considering that the final result is just adding a color to a link.
A different solution would be customizing their signature with links and/or colored text, and signing their name (~~~~) whenever you want to include a more styled link to their user pages. Another problem with using images containing text is that they cannot adjust the text to the line height; though that's how a signature works anyway.
Though, it's pretty kind of you to ask users if they're interested in colored links. So i'd say that they should say "thank you" even for a single-colored link and be happy with what they got.