Paint with Pixels is a very simple app to create pixel art with your iPad. Create hundreds of colors and tones mixing pixels from the palette. Each pixel has a low opacity that will allow you to mix your own colors and play with color theory. Tap a red pixel then a blue pixel and you will get a purple one. The more you tap the darker the pixel gets.
Here is a promo code for Paint with Pixels: LL3AKWNFK9EM