Guitar vs Keyboard Amps?

A Keyboard amp or a Acoustic guitar amp should send out a clean and noncolored sound while a Electric guitar amp is designed to be used overdriven and give a colored and distorted sound.

An Amp designed for acoustic guitar should be possible to use with a keyboard/piano

I think that keyboard and acoustic amps are designed to be totally clean, so that the full range of tones come from the instrument. A guitar amp is designed to leave the clean and move into the dirty regions, i.e overdrive and distortion - it is intended to play a role in creating the overall tone, whereas the others aren't.

I am not totally sure about this but it is also my understanding that guitar amplifiers don't cover the sound spectrum as much as keyboard amps. Guitars are really midrange instruments relative to piano's and voices with have greater range. PA's are full spectrum amplifiers also.

Here is an article that explains some of the differences between electric and acoustic guitar amps and speakers. A keyboard amp should work very well for acoustic guitar, they are very similar.

An electric guitar amp is not suitable for amplifying an acoustic guitar. These amps are essentially "lo-fi" amps. Many use circuitry that is designed to distort in a manner that is pleasing in an electric setting, but not so in an acoustic setting. All have large speakers that are essentially just "woofers," incapable of reproducing the high harmonics that give the acoustic guitar its characteristic sheen and brilliance. Also, guitar speaker cones are constructed differently from hi fidelity speaker cones; the cone material and speaker surrounds (the material connecting the cone to the frame) are designed to encourage "break-up modes" in the motion of the cone that create a characteristic speaker distortion that is pleasing to electric guitarists.
In performance, it is likely that the audience will hear the guitar through a high-fidelity PA system provided by the venue, so the player need not be concerned about power amplification. However, one may wish to carry a power amp for monitoring, or for performing in a small venue. The cheapest option is to use a keyboard amp in conjunction with whatever preamps and effects you may be using.

A more self-contained approach is to use an amplifier designed especially for amplifying an acoustic guitar. Several of these are now available. They combine some or all of the features of a good preamp with a power amp and wide-range speakers. They often also include extra equalization for controlling feedback, and reverb or other effects. Some include an additional mic input so that vocals can be combined with the guitar sound.

What's the difference between a "Keyboard Amp", an "Accoustic Amp" and a "Guitar Amp"?

In addition to what's already been mentioned, keyboard amps often have multiple inputs. Which can be handy.

And some keyboard amps may require a slightly higher input level than an accoustic amp. If you have a pre-amp or active pickups then that probably won't matter, but it's something to keep in the back of your mind.
That said, I've plugged a mic straight into the input of a keyboard amp and used it for a low-budget PA in a small meeting room (~50-75 people) for spoken word stuff - no instruments or singing.

Thanks everyone!

I'm looking at the Roland KC-150. Given what everyone has said, it sounds like that'll do just fine.

Thanks again.

Thanks everyone!

I'm looking at the Roland KC-150. Given what everyone has said, it sounds like that'll do just fine.

Thanks again.

Hey there,
I've actually had the pleasure to use a KC-150 for acoustic guitar and for keyboard(my school owns one). It's a very nice, loud amp with a nice sound to it.