Unsubscribe automatically occurs when next or completed is sent. If you wan to unsubscribe manually, then use,

RACSignal *backgroundColorSignal =
  [self.searchText.rac_textSignal
    map:^id(NSString *text) {
      return [self isValidSearchText:text] ?
        [UIColor whiteColor] : [UIColor yellowColor];
    }];

RACDisposable *subscription =
  [backgroundColorSignal
    subscribeNext:^(UIColor *color) {
      self.searchText.backgroundColor = color;
    }];

// at some point in the future ...
[subscription dispose];

Avoid retain cycle:

@weakify(self)
[[self.searchText.rac_textSignal
  map:^id(NSString *text) {
    return [self isValidSearchText:text] ?
      [UIColor whiteColor] : [UIColor yellowColor];
  }]
  subscribeNext:^(UIColor *color) {
    @strongify(self)
    self.searchText.backgroundColor = color;
  }];

Notice! Using instance variables within blocks will also result in capturing self inside block. You can turn on a compiler warning to alert you if your code results in this problem.