There are many quick ways to do tonal studies. I used to use Derwent Graphitone and Copic markers. And recently I've been using the Noodler's Lexington Gray fountain pen ink that's filled in a Pentel Colour Brush Pen.
Each medium has its own characteristics.
Derwent Graphitone is water-soluble graphite so you'll need water to activate the solid graphite. It's cheap and convenient to use with a water brush.
Copic markers are more convenient because you can just uncap and use them. They dry very quickly. Downsides are they are expensive, and since the ink if alcohol based they may bleed to the back of the page, and there's a marker for each shade of gray.
Lexington Gray ink is my favourite medium currently. It's cheap. You can get a 90 ml bottle on Amazon for around US $13.
And when you put the ink into a brush pen or waterbrush, it's incredibly convenient to use.
You can also dilute it to different tones.
Here's a sketch that I drew with pen and ink, and painted over with Lexington Gray.
The paper you use can affect how dark the ink will look. The sketch above was drawn in the Strathmore 500 series mixed media journal which uses 100% cotton paper. The paper is very absorbent and takes in a lot of ink and hence makes the appear darker.
On paper that's less absorbent, the ink will appear lighter. The paper above is from the Leda Art Supply sketchbook.
In the close up above, I've painted the sign board with diluted ink, then glazed over with the non-diluted ink. Just like watercolour, when the surface is still wet, the ink will diffuse into the wet areas.
The ink is waterproof and permanent when dry so it's suitable for use with other media.
The window (pointed) above was painted with two layers of ink and the wall with one layer. Applying multiple layers of ink only makes the ink slightly darker. It's not going to be anywhere near black so it's good to have black ink as well.
White gel pen works great with Lexington Gray. The contrast is quite good.
Another advantage of using the ink in a brush pen like this is, the application can be very even. It's easy to create a flat wash. And if your brush has large bristles, you can cover large areas easily.
So with black, white and a few grays, you can do tonal studies very easily. This is very economic and it's a convenient way to practice.