I have been experimenting with hooking up low-cost ESP8266 WiFi modules to the Radio Shack Color Computer (or anything with an RS-232 port) using cheap RS232-to-TTL adapters. Most of these adapters only support 3-wire RS-232 (transmit, receive and ground). This is fine for most uses of the CoCo’s built-in Serial
Here are some more options for getting an old 1980s computer on the internet via RS-232 to WiFi/Ethernet adapters.
The ESP8266 developer modules, like the one I picked up from Amazon for $8.99, are available for around $2.75 when shipped from China. That, and an RS232 converter for about .67,