Appropriate filter steps for resolving a droplet pixel
First simple thresholding is usually not a good idea. However, if you want to do that, I suggest to also consider other color spaces. In your case, HSV might be a better choice; You can convert your image into HSV-image then you can define a range on Hue channel to select yellow.

Also, take a look at this article; it is about implementing Adaptive Thresholding.