This is how I create halftone dots for illustrations.

Step 1. I usually start with an ink drawing. I create a layer of white underneath and set the inked layer to multiply. I then create a new blank layer and place it under the line layer and over the white layer.

Step 2. Staying on the blank middle layer, I create an awesome shape (as shown here) with the lasso tool. After I have the shape I use the gradient tool to make some kind of gradient.

Step 3. I copy the layer with the gradient and paste it into a new, blank file.

Step 4. Because the colors are usually light, I make them darker. Mess around with it how you like, but it usually works better with a darker shade.

Step 5. After converting the image mode to grayscale, the option to convert to a bitmap becomes available. When I choose bitmap mode, this screen shows up. Under the "Method" section it usually says 50% Threshold. I then change that to Halftone Screen. Another set of options will show up, those can be adjusted for whatever kind of look you are going for.

Step 6. My bitmapped image will usually look something like this. Sometimes I like to have the dots larger. That can be adjusted in the options when converting to bitmap mode. I often undo the conversion and adjust a few times before it looks right. (Just a word of advice, I keep the same frequency of dots for all the halftone dot gradients in a single image. The pattern seems to work better that way.)

Step 7. In order to get rid of the white pixels I use this process learned from Scott Franson at BYU-Idaho. It leaves the layer with only black pixels and everything else is transparent.

Step 8. Now, to easily change colors I click the "lock pixels" button for that layer (circled in red). I then hit the option key + delete to fill the pixels with whatever color is in the foreground.

Step 9. Now I have the halftone how I want it, so I copy and paste it back into the original file. Placing the layer under the inked line drawing layer.

Step 10. I usually have to move it around to get it exactly where the gradient was before. Then I just delete the old gradient layer. If you do this right, you can do many halftone gradients and line up the patters so they fit together flawlessly.