They will also be equally vulnerable at the first few kilometers on the beach wearing those Smurf suits if not more so, and then even more when they hit the tree line. The beach is the less defensible area and is never ever the spot where any reasonable army would feel comfortable in.
And there is not way the PLAN will be parking their frigates and destroyers all right off the beach exposed to every single anti ship weaponry the defenders can throw at them. Fire support via naval and artillery is always non line of sight, that is fact. There maybe some instances of amphibious landing ships providing fire but those counts more as direct fire support of which IFF identification issues is on another level entirely to say the least.
One on one armoured divisions, sure. But the PLA Marines don't have armoured divisions. Their vehicles are thin-skinned and highly vulnerable to anything larger than 7.62mm. The ROC defenders on the beach, with their mortars, .50 cals, field guns, bazookas, light tanks, and fortifications, are in a much more advantageous position. If the PLA Marines were to have a chance of taking the beach, they would've needed to minimise their losses coming in from the sea and maximise the amount of effective fire-support that their navy could've provided them. Both priorities necessitate the blue camouflage. Of course, this is 20th century PRC vs ROC. Nowadays, I'd be surprised if the ROC can inflict even triple digit casualties on PLA forces.
To keep this discussion from getting off topic I will keep this as straight as possible. The PLA Marines have at least several amphibious light tanks like the Type 63A back then and the ZTD-05 nowadays. Both of which can at least withstand small to medium arms fire, and the PLA would have certainly conducted serious off shore bombardment before hand to weaken coastal defenses before the actual assault. The defenders may have the more advantageous position, but the PLA Marines would have naval and air support before they even consider an such an attack.
Minimizing losses coming in from the sea is not going to mean much if you are going to maximize your chances of being spotted and attacked by the enemy, which is exactly what wearing extremely contrasting colousr vs the back ground is gonna get you.