I'm pretty sure this is "The Eternal Now" (1944) by Murray Leinster.
Dr. Harry Brett is meeting with a man named Hunt who introduces Brett to his niece Laura Hunt. At that moment they are suddenly transferred into frozen time, awakening elsewhere to an oddly-lit grayness.
It turns out that Brett had invented a mass-nullifier that, as a side-effect, almost infinitely speeds up time. His assistant, Dr. Cable, has stolen the idea and had been using it to steal things only to discover he couldn't return himself to normal time. Cable thus kidnaps Brett to fix his nullifier so he can return to normal time.
The start of the story speaks of the colours and sounds of everyday life:
THERE was sunlight. There were colors. There were noises. They stood in a perfectly normal office, on a perfectly normal afternoon, in a perfectly normal world.
As compared to the endless silent grey in the almost-infinitely-speeded-up instant:
Everything was gray, and everything was silent, and there were no shadows.
What they perceive as light to see by is gravity waves:
As the time-rate went up and up, Cable had said, there would come at last a ghostly gray light which would be that of the infinitely short vibrations which are gravitation.
The vastly accelerated release of energy from a nullified piece of radioactive material would be devastating:
“It’s a matter of radio-activity. Radium has a half-period of about two thousand years. Uranium’s is infinitely longer, five times ten to the ninth — five billion years. Suppose Cable wanted to get even with the normal world in which he never cut a figure? Suppose he brought radium into accelerated time? What would happen? It’s always three degrees hotter than its surroundings. It’s always giving off heat. Suppose its time-rate were accelerated so that its half-period became a fraction of a second of normal time?”
[...]
"Or he could use uranium,” said Brett more sternly still. “It might not seem as bad. But where there are milligrams of radium there are tons of uranium to be had. If he brought a mass of it into our time it might not create an explosion in normal time. It might disintegrate at what would seem a leisurely rate. It might take a year to destroy itself. But even a milligram of radium is nothing to play with, and this would be two thousand times as deadly. Do you know how much a milligram is? If you crush three aspirin tablets and divide the powder into a thousand parts, each part will be a milligram. A milligram of speeded-up uranium would make a nearly incurable bum in a fifth of a second, and there could be tons of it! If Cable put masses of that stuff about New York, nobody could approach it. No metal shield could stop its rays. Its radiation would make the very air radio-active, so that a hundred miles away you might breathe in poison which would sear your lungs.
