White Hot Spikes is the 3rd upgrade in BTD4 and the 2nd upgrade of path 1 for the Spike Factory in BTD5. In BTD4, it costs $300 on Easy, $350 on Medium, and $380 on Hard, and in BTD5, it costs $765 on Easy, $900 on Medium, $970 on Hard, and $1080 on Impoppable. It enables the spike factory to produce road spikes that can pop both Frozen Bloons and Lead Bloons. The spikes turn red at their tips.
It is recommended to get the first two upgrades of path 2 (Faster Production and Even Faster Production) before getting this upgrade, as producing spike piles at a faster rate is usually more valuable than allowing them to pop lead bloons.
This upgrade pairs well with Spike Storm, as it allows each spike storm ability to pop D.D.Ts.
The upgrade button in BTD4.
White Hot Spikes in BTD4
White Hot Spikes while the Spike Factory is out of range of the track, in BTD4
From left to right, a 2/0 spike factory and a 2/2 spike factory
Even though it's called white hot spikes, and the artwork shows spikes that are white at the top and orange at the bottom, the spikes actually appear reddish-orange in the middle with black at the bottom, although the very tips are white.
MOAB-SHREDR Spikes have the picture of white hot spikes on a MOAB even though this upgrade can be reached without buying the white hot spikes upgrade (e.g. 1/3 spike factory).
The bottom of the spikes aren't hot, because they are black and not red or white.