Try this: Take a pencil and mark the level of the water in the tank at rest. Turn off the water supply at the wall valve. Leave it overnight (i.e. at least 8-10 hours). Go back and look. Is the water level lower? If so, water is leaking from the tank into the bowl, regardless of whether your color test worked.
Sometimes, people color test but just drop the color in the tank without swirling it around to make it nice and dark and uniform. This lazy way should probably be sufficient if the water is leaking out of the bottom of the tank (i.e. through or around the tilt-flush). However, if the source of the water is higher in the tank (i.e. a cracked overflow riser or water running over the top of the overflow riser because the water level is set too high or the fill valve isn't turning all the way off), then the water will still flow but because the tank water isn't a uniform color, you won't see it in the bowl.
Assuming the tank water level is lower in the morning, then I would check:
The tilt-flush again. Most likely cause.
The overflow riser. Wherever the tank water level fell to, look right there at the overflow riser. Is there a horizontal crack there? Is there a vertical crack there? Pinch the overflow riser and see if it doesn't reveal a crack starting at the top and running down to the level where the water stopped draining. Sometimes, such a crack emerges under the clip for the refill hose, which disguises the crack. Sometimes, the overflow riser cracks at the intersection of the riser and the tilt-flush base. If you have a crack there, the water will drain all the way down to that crack, but it may be hard to distinguish between it and a faulty tilt-flush base.
One other possibility is that you (or someone else) has/have shoved the refill tube from the fill valve into the overflow riser. Big no-no, because it can siphon the tank to the point where the end of the hose sits in the overflow riser. Make sure it is daylighted above the top of the overflow riser on a little clip that should have come with the fill valve.
Those are the obvious issues. If one of them doesn't appear to be the problem, post a picture of the inside of the tank at rest; i.e. when the fill valve shuts off. You would be surprised how much we can diagnose just from looking in the tank, or at least suggest areas of investigation.
Good luck, and do let us know if it turns out well, or come back for more if it doesn't.