After the image loads, select a color from the Colors list to paint with that color. You will see the Current color in the bottom left. You may need to left-right scroll in the Colors list section to see more colors. If you're having trouble scrolling with the mouse/touchpad, you can also use the left/right arrow keys to scroll, and the up/down arrow keys control the scrolling speed. You can either click on the color to select it or type on the keyboard the number within the color. Note the backspace button also works when typing numbers. The number below the color represents the remaining pixels to be colored in the image.
After selecting a color, click on the boxes in the Image section. This is where you will be coloring. You can click and drag if you want to color multiple boxes with your Current color. To make the game easier, you can right-click to automatically select the correct Current color and color the box. You can also hold the control key while clicking to color a group of adjacent boxes. You can also click on the - or + buttons to zoom in/out. Typing - or + on the keyboard also works. The Image section is usually large, so it can be scrolled left-right and up-down. If you're having trouble scrolling with the mouse/touchpad, you can also use the WASD keys to scroll, and the up/down arrow keys control the scrolling speed.
The coloring does NOT auto-save, so if you don't want to lose your work, then click on the Save button to save your progress. It'll save a file that you can load in the future. Note the save file is only compatible if you use the same image.
The last section is the Preview section that shows your selected image. There is nothing you can do with this, as it is just a preview for your image.
When you manage to correctly color all the pixels, there will be a Victory button to showcase your coloring.