Those are amphibious fighting vehicles, used for landing from the sea. The sea, as we know, are blue colored.

These are amphibious?

http://ichef.bbci.co.uk/wwfeatures/1600_640/images/live/p0/3...

These are not. But they belong to the same unit of those amphibious light tanks, so used the same camouflage. During a landing operation, these long range rockets are likely deployed on the decks of flotilla as artillery support for the beachhead assault.

Those are transporter erector launchers for the YJ-62 anti-ship cruise missile. They are deployed as shore-based defensive batteries. The coloring is likely intended to make them blend in with the sky (when viewed from the ocean surface) and the surf zone (when viewed from above).

As for deploying rocket artillery on ships, that wouldn't be a good idea. Firing platforms need to be better secured in order to be used from a ship. Regardless, China doesn't have many amphibious assault ships with significant clear deck space.

There's a section about it in TFA:

"Another conundrum with the modern camouflage design seen during China's Victory Day parade is that the vehicles feature bright blue painted blocks. Why? Some experiments have been done with bluish tones to break up the shape of vehicles against the sky on flat terrain, or for use in icy or snowy areas. “If the vehicles are amphibious then they might have some concealment during their water-based maneuvers, but the trade off for being highly conspicuous on land is not worth having it permanently,” believes Guy Cramer of Hyperstealth Biotechnology. In this case, the blue scheme is likely there for decoration. It certainly made the parade a spectacle, but further muddies the murky world of military camo. —JS"

Makes it harder to spot them against the sky.

There are all different colors of camo depending on the environment it will be used in.

Where are tanks going to be used that bright blue is a good idea?

In the sea. When conducting a beachhead assault, these amphibious light tanks will come out of some giant amphibious assault ships, which are typically parked far away from the beachhead so enemy on land cannot see them. So the effect will be that a swarm of floating tanks suddenly appears and quickly approaches the land. This concept is obviously designed for an island invasion scenario.

I guess blue could be a good color for a hull-down vehicle.