Primary and secondary colors
In Pixly, we have two colors, the primary and the secondary. Often called by the names of foreground and background colors too.
The primary color is the one on the top, and the secondary the one on the bottom.
SOME tools allow you to swap the colors, in this case, the one the yellow arrow is pointing to will act as the primary, the other as the secondary. To swap them, just touch the yellow arrow.
The concept is quite simple, most of the time, the SECONDARY color should be set to be the same as the BACKGROUND color of your artboard, and the PRIMARY color should be set to be whatever you want, whatever color you'll be using to paint.
So what's the matter, why a whole page just to describe this? It turns out that (for a simplified interface), the secondary color sometimes have another purpose, and you might have to change it back and forth.
E.g.: The gradient tool will make a gradient that ranges from the primary color to the secondary color. You probably will want to change the secondary color to some color that you think that matches awesomely with your foreground color, and then will place your gradient.
The catch here is: You MUST remember to switch the secondary color back to the color of your background.
MUST is kinda subjective here. Unless you're using filters, or need to see the onion skins properly, or use the eraser, then probably you're fine if you delay switching it. But as soon as you need one of these, you gotta change the secondary color to the background color.
With that said, if your background is transparent, the secondary color is mostly irrelevant and you can use it in whatever way you wish!