It's likely you're thinking of the English word "cyan", which refers to the color blue-green. However, there isn't a direct translation of "cyan" in Welsh.
If you are looking for a Welsh word for a specific shade of blue-green, you could use:
* Glas tywyll - dark blue (for a darker shade)
* Glas gwyrdd - blue-green (more literal)
It's important to note that Welsh doesn't have as many specific color terms as English, and shades of blue-green would often be described with a combination of terms like "glas" (blue) and "gwyrdd" (green) depending on the specific shade.