Opencv color segmentation of traffic signs: HSV ranges
To those looking at this in the future, I realized I was being silly and forgot how HSV worked! To pick up the yellow more so than the green I needed to heighten the threshold of the saturation value within the range!

As a side note, I also was able to "normalize" the colors of the image a bit more by converting to Lab color space and applying CLAHE (for localized histogram equalization) and as well on the saturation values within the HSV color space. This allowed me to get the signs in ideal lighting conditions almost 100% of the time (roughly speaking of course) with very little false positives, but didn't really alleviate my problems on images with a redish/orangeish hue as seen in the second example image. But, as was mentioned by harshkn, this approach is very simple so I suppose I can only ask for so much success! Thanks to all who replied with suggestions!