Direct thermal technology is used mostly for purposes that do not require colored labels or where non-black colors can render tags less machine-readable. Barcodes and QR codes are examples. Therefore, having multiple font colors is not critical when buying a direct thermal printer.
Note, Brother is the only one in the industry that sells two-color direct thermal printers. However, those models print narrow labels and are not appropriate for making standard shipping tags. Also, multi-color direct thermal labels are more for organizational than decorative purposes as they lose readability after a few years. If you need fade-resistant colored tags, you can use a thermal transfer, inkjet or laser printer instead.