When pixedit first starts, the cursor changes to a crosshair. Click on the screen pixel whose color and whose colormap you wish to edit. Then, in the upper right there will be a box containing the current color along with a label showing the current RGB values in X11 hex format.
To adjust the current color you can use one of the nine sliders, each controlling one of the RGB, HSV, or CMY values for the current color. Or you can click on a cell displayed in the palette to use its color. The palette button underneath the hex label switches between the three palettes: range, narrow, and wide. You can also click on the hex label to directly enter new RGB hex values.
If you click on the "match" button, the palette will change to the colors in the file /usr/lib/X11/rgb.txt which are closest to the current color. As you select these colors, their names appear below the bottom buttons. Use the palette button to return the palette to an ordinary selection.
The "select" button allows you to choose another cell to edit, the "cancel" button restores the current color to its original value, the "restore" button restores all the cells of the colormap to their original value, and the "quit" button exits out of pixedit.