Instead of head or tail in the above illustration, you have red or blue, there is a 0.3 chance of red, 0.7 chance of blue. Then extend the tree to 4 further branches, red and blue under the initial red branch, and red and blue under the initial blue branch. Each red is 0.3 and each blue is 0.7. Then you're multiplying the probabilities, ie 0.3 x 0.3; 0.7 x 0.3; etc. Difficult without drawing lines in felt pen over my laptop screen!!
Hard to explain without drawing it but I will have a go you start off with two branches representing the outcomes that can happen when you pick the first cube. then you branch off from each of the two branches to give you the outcomes that can happen when you pick the second cube so possible outcomes are Blue Blue , Blue Red, Red Blue and Red Red