beginner question : V-lookup problem

Hi ,

I am a beginner to Quantrix.
I’ve read many lookup function like select / using as in the website.
However, when I was trying to do something like excel v-lookup. I have no idea at all :
Matrix 1 with category Colour :
Red 1
Blue 2
Green 3
Yellow 4

Matrix 2 with lookup category:
Blue x
Green x

I type using as / select formula and try to get blue x = 2 and green x = 3; however, both of them do not work.

Anyone could give advice me on this ?