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To collect riffle shrimp, find a riffle. Sounds obvious I know, but many people don't realise that these guys prefer the fastest flowing water you can find, i.e. riffles. The immediate downstream side of the river crossing in your first pic would be perfect as would the riffle area a little upstream (second pic). Find a suitable rock to flip over i.e. don't get a hernia trying to lift the biggest rock in sight. don't neglect little rocks either because they may have caves underneath and riffles will hang here in numbers. hold your net downstream of the rock you are going to flip up. Lift the rock. Thats it. The riffles will usually swim with the current straight into your waiting net once they realise the rock is moving. However it is worth scooping in the area under the rock as they sometimes will sit still. You will need to travel to different areas of the creek as areas are often dominated by one sex or the other. Also as good practice, you should immediately return berried females unharmed. Remember to comply with all fisheries and state legislation concerning collection of wildlife. This includes not collecting wildlife from inside National Parks (unless you get permission/permit, which you generally won't).

I have a couple of theories to suggest why this may occur. 1. Development of the eggs. Eggs will change colour depending on the development of the embryo inside. (See Kitz post above, two weeks difference in maturity). This is easily seen in fish eggs where larvae inside the egg will darken over time. 2. Maternal provisioning. This refers to what the mother shrimp puts in to the eggs. For example, the amount of lipids (fats) put in eggs will depend on many factors, including how many eggs the female produces (more eggs means less lipid per egg), the health of the mother (sick/stressed mother will keep more lipids for herself), the mothers diet (how much lipid she can gather from the environment), genetic variability (some mothers are better at getting, storing and provisioning lipids) and many other factors. Now if we assume that egg colour is due to some colour producing chemical (e.g. Carotenoids) then the same rules apply. Some mothers can give their eggs more, some less. More chemical equals more colour, less chemical equals less colour. 3. Genetic variation. It could be that individuals could produce different coloured eggs due to differences in their genetics. For example, some chickens produce green eggs, others blue and some white. Interestingly this is also affected by the mother chickens diet, so you can get some normally white egg laying chickens to produce blue eggs by adding certain chemicals to their diet (essentially a combination of theory 2 and 3).