By using the Marquee tool, you can actually create this dithering technique in about half the time it would take to do both sections manually. I’ll try to explain the process using text wording, but it would be easier to learn it by watching a video of the process.
Copy the completed bottom half gradient area, and then paste it at the top. From the filebar menu, select Sprite–>Rotate Canvas–>Flip Canvas Vertical.
Without removing the marquee, apply flood fill (Paint Bucket Tool) with a 3rd unrelated color to the 2-tone selection, making sure that “continuous” is not selected.
Next, flood fill the remaining color to its opposite (i.e. cyan to purple, or as the case may be, purple to cyan). Then flood fill the 3rd color to the remaining color of the original 2 tones. Volia! There you have it.
Reposition the marqueed section to align to the original gradient section, so that the bottom edge of the mobile section meets flush with the top edge of the original gradient section.
Finally, remove the marquee. You may need to fill in a single column of pixels to complete the pattern, depending how the copied marqueed section was aligned horizontally to the remainder of the background.