If RED is coded as UHG, then BLUE will be coded as:

Let's determine the pattern between RED and UHG by looking at alphabetical positions:
* $\text{R}(+3)\to \text{U}$
* $\text{E}(+3)\to \text{H}$
* $\text{D}(+3)\to \text{G}$
Each letter is shifted forward by 3 positions. Applying this to BLUE:
* $\text{B}(+3)\to \text{E}$
* $\text{L}(+3)\to \text{O}$
* $\text{U}(+3)\to \text{X}$
* $\text{E}(+3)\to \text{H}$
Thus, BLUE is coded as EOXH.