SG-1000
The original SG-1000 only features RF out, typical of the era. All Japanese SG-1000s feature a TMS9918 VDP, which outputs composite video that goes into the RF modulator. It is possible to modify an SG-1000 with composite video using a simple amplifier circuit, tapping the video signal where it comes into the RF modulator. Theoretically it is also possible to add a TMS-RGB board to an SG-1000 for RGB output, but this requires significantly more work, as the VDP needs to be swapped with a TMS9928 (which outputs luma and color difference signals) and an external oscillator has to be added to provide the 3.5795 MHz clock signal for the system. Some rewiring also needs to be performed on the SG-1000 motherboard and the console shell needs to be modified with a proper connector. One board revision of the SG-1000, the 171-5078, can be fitted with a TMS9928 and even has footprints for a dedicated main clock circuit to help facilitate this swap.
A TMSdigitizer board can also be added for LumaCode support, which can provide video quality on par with most RGB mods and does not require a VDP swap. However, this requires the use of an external device such as an RGBtoHDMI board or an OSSC (Open Source Scan Converter).
SG-1000 consoles released under the Grandstand label in New Zealand use a TMS9929 VDP which is the PAL equivalent of the TMS9928. It should be possible to modify these with a TMS-RGB board without too much work, and composite video can be added the same way as the Japanese SG-1000. These are also compatible with the TMSdigitizer.
SC-3000
Unlike the SG-1000, the SC-3000 features both RF out and composite video with mono audio, the latter via a 5-pin DIN connector. This connector follows the exact same pinout as the Sega Master System and Genesis/Mega Drive so any AV cable for those systems will work. French SC-3000s go a step further and feature analog RGB output, which can be connected to an RGB monitor or upscaler with a simple cable. PAL SC-3000s have composite video like their Japanese counterparts.
Adding a TMS-RGB is also possible on the SC-3000, with some difficulty on Japanese models as they use a TMS9918 and require a VDP swap like an SG-1000. PAL models use a TMS9929 so adding a TMS-RGB is much easier on these systems. LumaCode support can be added with the TMSdigitizer as with the SG-1000.
SG-1000 II
All SG-1000 IIs only have RF out like the original SG-1000. There are three revisions of the system, one which uses a TMS9918 like the original, another which uses the TMS9118, and a later revision which incorporates the VDP and PSG chips into a single ASIC. All video mods for the original SG-1000 also apply to the earlier two revisions of the SG-1000 II, and they also require the same amount of work when it comes to adding a TMS-RGB board.
The ASIC in the later SG-1000 II features unused analog RGB outputs along with composite sync, and these signals can be fed into an amplifier circuit to generate usable RGB video. Although the video quality is generally good, the color palette is somewhat different compared to an actual Texas Instruments VDP, similar to how MSX1 games appear on an MSX2 system.