It looks to be an issue with UIToolbar.
Instead of setting the tint color, you can add a CALayer with a background color with an alpha component less than one, depending on how prominent you want your color to be, like so:
CALayer *extraColorLayer = [CALayer layer];
extraColorLayer.frame = CGRectMake(0, 0, toolbar.frame.size.width, toolbar.frame.size.height);
extraColorLayer.backgroundColor = [UIColor colorWithRed:r/255 green:g/255 blue:b/255 alpha:0.5].CGColor;
[toolbar.layer addSublayer:extraColorLayer];
This appears to create the effect you're looking for.
Adapted from this answer.