Many reverbs have some onboard EQ. Depending on your host, you may also be able to insert a separate EQ plugin before or after the reverb.
Generally, you can think about EQing reverb as three bands:
Lows: Reduce or cut the lows to cut down on mud and boom. Getting surgical here can sometimes really help clean up a mix. Increase the lows for special effects – running a kick drum through a low-heavy reverb will give you a tasty huge BOOOOOM!!!
Mids: Reducing (dipping) the mids of a reverb signal can thin it out, sometimes helping it fit in the mix. It can also help give you a very “hi-fi” sound. Boosting the mids (relatively) can increase thickness and body.
Highs: Reducing the highs can go a long way to cleaning up annoying sillibance in vocals (“s” and “t”) if the reverb is catching too much of it (also think about using a de-esser on the vocal channel itself too). Reducing or cutting the highs can also make the reverb become less noticeable overall, which may sometimes help it sit in the mix better. Boosting the highs can work well when you want to emphasize the the reverb (make it noticeable) without muddying the mix.
You’ll notice that I’ve used terms like “sit better in the mix”. This is an artistic judgment you have to make (if you cut everything to make the reverb sit “best” in the mix, you won’t have anything left!), and you’ll have to make it in the context of the mix.
You’ll also notice that I’ve given advice for reducing AND boosting different frequency areas. There’s no simple advice like “Doing X will always improve your mix” (even the famous 500Hz dip!). Techniques will have certain audible results, but you have to decide if those results are appropriate for your mix, for your music.