In many ways, the CoCo 1 and 2 reminds one of the Apple II+. Both machines really had a widespread maximum of 64KB of RAM. They used 8-bit processors running at speeds close to each other. Both machines can produce low resolution direct colors but really show detailed color images with NTSC composite artifact colors. If you subtract the purple/green combination from the Apple II, the graphics of a CoCo and an Apple can look very, very similar. Both machines had somewhat limited (pre-IBM layout) keyboards and did not support lowercase characters officially. Both came with ports for analog joysticks and cassette storage. The sound hardware for each machine was rather crude and neglected. Disk storage was only slightly better on the CoCo.
The Color Computer was never quite as successful as Tandy would have liked The graphics and sound were very unimpressive compared to the king of the 8-bit computers, the Commodore 64. Before the C64, it was rather lacking against the Atari 400, even if it did have a somewhat better keyboard. Even though it was similar to the Apple II in many ways, the Apple II had a full expansion bus whereas the CoCo only gave a single cartridge slot. (You could expand the cartridge slot by four slots with a Multi-pak interface, but the disk drive's controller was plugged into a slot). Software was somewhat limited and many of the games either looked like Apple II ports or were edutainment or early-children titles. Larger publishers stayed away, especially after the home video game crash essentially destroyed any remaining market for cartridge-based computer software.
Sierra Online and Tandy were firm business partners. Sierra had good reason to show some support to Tandy's lesser computers given all the assistance Tandy gave Sierra just after the video game crash. Its edutainment titles, Donald Duck's Playground, Winnie the Pooh in the Hundred Acre Wood and Mickey's Space Adventure only required the capabilities of a CoCo 1 or 2 with 64KB, Extended Color BASIC and a disk drive. These games looked and played very close to their Apple II counterparts. Given that it was not especially difficult to port these games and the kid-friendliness of the CoCo, Sierra's choice to support the CoCo with these games was reasonable business decision.
Sierra then decided to become a bit more adventurous. It released Silpheed and Thexder from its Game Arts Line. These games were ported over to the CoCo by Syngeristic Software. Silpheed supported both the CoCo 1/2 and CoCo 3. The CoCo 1/2 capabilities could not do Silpheed justice, so it was pretty poorly received. There were some enhancements for the CoCo 3, such as using 320x192 16-color graphics, but the graphics and gameplay were drastically simplified compared to versions for other home computer systems. There is no music, no introduction and the sound is not going to give any other system a run for its money. There was only so much you can do with a 16KB cartridge that has to contain two versions of a game.
Sierra's next game was Thexder. It also came on a 16KB cartridge but only ran on the CoCo 3. It uses a 320x192 color mode with 16 colors. The original game came with 16 levels but this port only has five. Some character designs were redone, animations simplified and there is no music. The title screen uses some really ugly color ASCII-style graphics. While a noticeable improvement over Silpheed, this is still a pretty poor port.
Finally, I would like to discuss Sierra's ports of its AGI games. It released King's Quest III: To Heir is Human and Leisure Suit Larry in the Land of the Lounge Lizards. Both are the only commercial games released during the CoCo's lifespan to require 512KB of RAM.
You might think that Sierra would release King's Quest I and King's Quest II before releasing King's Quest III. It is rather unusual to throw new players into the third game in a video game series. However, King's Quest III is quite unusual in the context of the King's Quest series. King's Quest II, IV and VI directly follow from the events in the respective previous game. King's Quests III, V and VII follow II, III and IV in a less direct or obvious fashion. King's Quest III seems quite divorced from King Graham and Daventry at first, so it is a pretty decent start to the series on the CoCo.
KQ3 on the CoCo looks like a game being played with an EGA card on a non-Tandy PC and sounds like it too, although there is some missing music and most of the sound effects are gone. It came on five double-sided disks for ten disk sides! You will be changing disks quite often, just like on an Apple II Mercifully, it does offer drop down menus, faster animation speeds and a clock. The clock is absolutely essential for this game and something the Apple II version did not feature. It has appropriate selections for color whether you are running the game on a composite color or RGB color monitor. However, movement without a joystick is done by using Ctrl and letter keys, which is very odd considering the CoCo 3 has a perfectly good diamond cursor key arrangement unlike the CoCo 1 & 2.
Leisure Suit Larry was another odd choice for the CoCo 3. LSL was an adult game and the CoCo 3 was targeted primarily at children. Moreover, RS was reluctant to advertise the game in its catalogs, Leisure Suit Larry did not appear in one until 1990 and the game hails from 1987! However, the game was one of Sierra's biggest sellers and required only three disk sides compared to the mammoth ten of KQ3. When you start up LSL you finally realize why there is so little music and so few sound effects in these ports. Sierra's CoCo 3 AGI engine could not handle animation and music at the same time, so music only plays where there is no animation on the screen. You can see this in Lefty's bar when you play the jukebox, all input and animation is suspended for the duration of the song. The Apple II AGI games had little music and sound, but they don't even have a DAC. However, the Apple II ports of Sierra's games also do not allow input or animation when music plays.
If you have a hard drive, you can install these games to a hard disk. The procedure is described in the reference card but it does require a command of OS9. After the CoCo 3's discontinuance in 1991, there would eventually be unofficial ports of the other AGI games to the CoCo. Undoubtedly they must be easier to play than the official ports. Obviously the official ports were not great sellers, Sierra did not make any more after these two. The CoCo 3 was discontinued in 1991 and Sierra had moved on from the AGI engine years before.