Is there another colour that would go well with a blue merfolk deck?

[Charliefen]

I was thinking of making a mono blue merfolk deck but want to know if its better to add another colour, if so what?

[Ghebert]

I would say think of what you don't have in the deck. For example, removal spells, board wipes, mana ramp, bounce, flicker, health gain, etc. and figure out which one you would like to have. I think blue and white merfolk deck would be good. Have a little control with {Oblivion Ring} and {Pacifism} maybe some other buff spells. {Planar Cleansing} maybe. The possibilities are endless.
But one thing, think about exactly what you deck's main achievement is and build around that.

[Trevor]

I think black for removal would be good. But white is another amazing idea. Also maybe red for burn

[BulgyCrimsom]

There's alotta cool white merfolk also some spells like {Summon the School} checkout lorwyn for them

[BlackJester]

mono-{B} merfolk can be few strong though.

[scarsabrex]

mono-{B} merfolk can be few strong though.

i hope you meant mono{U}

[BlackJester]

mono-{B} merfolk can be few strong though.

i hope you meant mono{U}

No way!  A whole deck built around the raw power of {Merrow Bonegnawer} and {Hollowsage}! 

Jokes.  Yeah, I meant {U}. 

[Dudecore]

People do {U}{B} Dreadfolk. Usually mono {U}

[Thorn]

Esper Merfolk!

[InfinitiveDivinity]

There's alotta cool white merfolk also some spells like {Summon the School} checkout lorwyn for them

This.

There are some pretty sick White Merfolk out there.

[adventus]

If you go WU definitely use {wanderwine hub}