What is this colourful insect with a large thorn?
As @Kiritee mentioned this is actually a treehopper, due to thier unusual shape they are of long interest of naturalists.
This is a female Umbonia crassicornis.
Here's a female with her colourful nymphs captured by
C. D. S. Molinari
.

Facts:

The females have a pointed pronotal horn (spike), the males' are blunt, with a flat top.

The adults display variation in colour and size.

The pointed pronotal horn is an adaptation as the insectivorous birds mistake them as thorns (on stem) and do not consume them.



This is a variable species as to size, color and structure, particularly the pronotal horn of males (which is more angled posteriorly than the females' and often somewhat expanded apically).
This tall, essentially perpendicular thorn-like pronotum discourages birds and other predators from eating it, if only by mistakenly confusing it with a thorn.
Typically, the adult is green or yellow with reddish lines and brownish markings.
The range of this species is from Northern South America all the way to Mexico and Florida. Its preferred hosts are ornamental and fruit trees of subtropical regions.


Parental care: The females on sensing an intruder would rush to her nymphs and would charge the offender by kicking her legs, fanning her wings and sometimes even buzzing. The females communicate with the nymphs to ensure that they are in a group.
References:
Encyclopedia of Land invertebrate behaviour, Encyclopedia of Insects and Libraries of life


Picture References:

A female with eggs.
Two males (blunt top)
Original size

Good reads:

Thornbug to Thornbug-The inside story of insect song

Libraries of life (audio clips)