ACDSee Pro is currently free (regularly $4.99)
I've used this app for a long time for adjusting light and color. It has the usual editing features you'd expect and also includes a good camera function. The 5-band Light EQ tool provides adjustment not only of blacks, shadows, highlights, and whites, but also of mid-grays, which is usually where I start my adjustments, especially for b/w. The Color EQ tool provides hue, saturation, and brightness adjustments for 8 color ranges, allowing much finer adjustments that the usual Saturation and Vibrance tools. It also has some useful filters and effects.
There are other editing apps that have these adjustments -- Pixelmator Photo, for one -- but for free, what's to lose?

ACDSee Pro
Capture, perfect, and share your photos with ACDSee Pro for iPhone. ACDSee Pro combines a professional grade camera and photo editor with all the tools you need to produce stunning photographs. Camera Harness the full power of your device's camera with specialized shooting modes, lossless RAW...
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I've used this app for a long time for adjusting light and color. It has the usual editing features you'd expect and also includes a good camera function. The 5-band Light EQ tool provides adjustment not only of blacks, shadows, highlights, and whites, but also of mid-grays, which is usually where I start my adjustments, especially for b/w. The Color EQ tool provides hue, saturation, and brightness adjustments for 8 color ranges, allowing much finer adjustments that the usual Saturation and Vibrance tools. It also has some useful filters and effects.
There are other editing apps that have these adjustments -- Pixelmator Photo, for one -- but for free, what's to lose?