A point to keep in mind is that water–in its pure state–is colorless. Water simply looks to be a certain color depending on the sediment it carries and, most of all, the colors it reflects from its surroundings. That accounts for the ocean looking blue in many parts of the planet, and for lakes and rivers showing various colors depending the the activity of the water (how much “stuff” it is carrying) and the surrounding shoreline.
I enjoy modeling water features–lakes, ponds, puddles, rivers, streams, waterfalls, etc. At one time or another, I guess I’ve tried every substance I’ve heard or read about to simulate water (which is, in my opinion, definitely superior to using real water, which doesn’t look all that “real” in a model railroading context).
I settled on Envirotex some years ago, and now use it almost exclusively. I’ve tried various other casting resins and so forth in the past, but really like Envirotex the best, by far.
If I’m modeling a pond or other similar still-water feature, I first assure that the bottom is completely sealed. As David noted, resins will find the smallest hole in your handiwork, and will most definitely end up on your floor if you don’t pay special attention to this step. Even then, it’s a darn good idea to place some kind of temporary/disposable cover on the floor under the area where you’re working.
I use acrylic paints to color the bottom of the feature. I start with black, or near-black, in what are supposed to represent the deepest areas, and then use various shades of browns, greens, and even yellows in working my way out toward the shoreline. I blend the colors together in a thoroughly random manner when I’m applying them so there are no well-defined separations between colors–they all blend together so you can’t really tell where one color ends and the next begins. I also try to lighten the colors a bit as I approach the banks, ending with a kind of light- to medium-brown at the shoreline areas that will be c