This is a great question, because you’ve given me pretty much all the information I need to figure out what you were looking at! Generally, if someone spots something in the sky that looks like it’s flashing, the first question we ask is “is it moving”, because if it’s moving, it’s probably a plane or helicopter, or a drone – something along those lines.
But your mystery object is not moving, and it’s not moving over the course of many nights. That eliminates another possibility, a nearby planet low in the horizon; planets are often extremely bright in the sky, but if you were looking for it every night, Mars or Venus would be shifting relative to things on your horizon, so you probably would have noticed. More distant planets like Jupiter or Saturn move much more slowly in our night sky, so they’re still possible culprits, but the other option is a very bright star. So we’re already narrowed down to a star or distant planet which appears low to the horizon for your mystery object.
The last piece of critical information you’ve given is that the object is low in the sky – this is what seals the picture. Very few astrophysical objects have intrinsic rapid flashes of color or brightness, which is to say that if we were observing them from the Space Station, from space, nothing out in the dark sky would appear to be flashing any colors. However, you are observing from the east coast of the US, not space, and the atmosphere is in your way.
The atmosphere is a major problem for astronomy. Most people typically assume that the atmosphere is generally transparent, and that light doesn’t interact with it in any major way. This is obviously false, if you think about it a bit more, given the fact that sunsets are due to light interacting with our atmosphere, but when the night is clear, and the stars are shining down, it’s easy to forget about our thin shell of air. Air bends light, and hot air bends light in a different way than cold air; it’s like having a series of lenses suspended above us, twisting and distorting the light on its way through. If the air is calm, these lenses made up of pockets of air don’t do much distorting. If the object is above you, the light from that star also has fewer air pockets to go through, so the light mostly makes it through intact.
However, if the light has to go through the atmosphere at a very shallow angle, as it does when the object appears low on the horizon, it must pass through a large number of these lenses. Because the atmosphere is only accidentally a lens, it doesn’t have to do a particularly good job of focusing of light, and it can produce a chromatic aberration. You may have heard this term from dealing with eyeballs or from dealing with fancy cameras; effectively it means that different colors of light are bent differently as they pass through a lens, and so certain colors wind up focusing in different places.
The atmosphere’s series of lenses are constantly changing, so the terrible focusing job that the air pockets are doing is also constantly changing. This results in exactly what you describe; a flickering effect, where the color of the bright object is changing, flicking between all of the possible rainbow of colors.