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

Let's establish the positional values of letters ($\text{A}=1,\text{B}=2,\dots ,\text{Z}=26$) to analyze the logic behind coding RED as UHG:
* $\text{R }(18)\stackrel{+3}{\to }\text{U }(21)$
* $\text{E }(5)\stackrel{+3}{\to }\text{H }(8)$
* $\text{D }(4)\stackrel{+3}{\to }\text{G }(7)$

The pattern shows that each letter is shifted forward by $+3$ positions.

Now, applying this exact same logic to the word BLUE:

* $\text{B }(2)\stackrel{+3}{\to }\text{E }(5)$
* $\text{L }(12)\stackrel{+3}{\to }\text{O }(15)$
* $\text{U }(21)\stackrel{+3}{\to }\text{X }(24)$
* $\text{E }(5)\stackrel{+3}{\to }\text{H }(8)$

Combining these letters yields the code: EOXH