Just showing the many colors that can be displayed in the Inbetweener app.  You can now choose any color available using the color picker and color wheels. So, can you color your animations, Yes. Should you use Inbetweener to color your cels/frames? If it's your only available option. The reason I say this that you may want to use vector lines for you animation or higher res raster images. GIMP is a free option, there's other options such as the free online tool Photopea. But, you can color your cels/frames in Inbetweener app if you really need to. I suggest that save often, ya never knew when disaster may strike.