Sand on the beaches of Kanyakumari, India
There are a few studies looking at different aspects of the magnificent colours observed at the Kanyakumari region. The colours, according to the research are due to the mineral content therein.

According to the article Hydrogeological Studies of the Coastal Areas of Kanyakumari to Colachel after Tsunami, South Tamil Nadu, (Perumal et al. 2010), it is common for zircon, rutile, ilmenite and garnet to be observed in the sand - this can acount for reddish hues.

In some places, monazite has been noted, also noted in radiological studies in Baseline Assessment of dose due to Natural Radionuclides in soils of coastal regions of Kanyakumari district in Tamil Nadu, India (Sarojini, 2014).

According to a Geological Survey of India (GSI) report of the region, there are also small amounts of sillimanite and leucoxene.  They also state that there is between 10-25% silica sand content. 

The presence of zircons, causing some of the brilliant lustre, was confirmed by XRD analysis discussed in the article Observations on the reported incidence of micro-diamonds in the beach sands of the Kanyakumari Coast, Tamil Nadu (Dinesh et al. 2011).

Much of the beach sand sources are likely to be weathered from the surrounding gneissic (including garnetiferous gneisses) and charnockites, as described in Perumal et al. and the GSI report.  Additionally, the report Post Tsunami Heavy Mineral Distribution
Between Cuddalore to Kanyakumari Along the
Tamil Nadu Coast, India – A Review  that also suggest some offshore and alongshore sources of the heavy minerals.