The bulb must have a "DOT" or appropriate SAE stamp on it. Currently, there are NO DOT approved "Red" bulbs.
Vehicle lighting equipment is covered in Chapter 547 of the Texas Transportation Code, Subchapter D. § 547.3215 - Use of Federal Standard. Unless specifically prohibited by this chapter, lighting, reflective devices, and associated equipment on a vehicle or motor vehicle must comply with:
The CFR will refer you to Society of Automotive Engineers (SAE) #J578, which will refer to several other SAE standards. Everything that complies with those standards is "legal".
Equipment manufacturers have to self-certify that they meet these standards. DOT does not test equipment unless they act against those who don't meet standards. You should insure that the equipment vendor is reputable and that all equipment has a DOT or appropriate SAE stamp on it (or if not marked, keep the box that it comes in which says it is "DOT approved") and under no circumstances use equipment that is for "off-road or show only".
For clear taillight lenses to be legal, they must consist of the following components:
Any device that impairs the required effectiveness of headlamps, tail lamps, reflectors, etc is prohibited. The lights, both front and rear, are made by the manufacturer to meet this safety standard. An alternate or additional material, particularly something dark, would impair its effectiveness. Additional information on lighting can be found in the Texas Transportation Code, Chapter 547, Subchapters D & E.
The inspection criteria for tail lamps states: "Tail lamp. Inspect and reject if: lamp is obstructed by any part of the body."
Transportation Code §502.409 prohibits attaching or displaying on a motor vehicle a number plate or registration insignia where: