Complimentary colours

Exert from Elements of Colour - Complimentary contrast
23-28 Mixture of bands of six complimentary pairs
29 Composition in the complimenary pair red/green and mixtures
30 mixture square of two complimentary pairs orange/blue and red-orange/green

As you can see from the varying mixtures of complimentary pairs there is another option for getting to either warm or cool nuetrals tending to either grey or brown.

If you squint at the sample above, which gives you the same effect as looking at the sample from a distance,   the individual colours tend to be reduced to nuetrals.  A very different way of obtaining greys and browns that is unique to tapestry.

Al this is probably more than you need to digest at the moments, but I thought that you would like to know this other option for grey.

On thinking about your choice in a logical manner I would recommend that you concentrate on the letter tapestry, taking the already started tapestry in for your tutor to look at, and being prepared to rewarp if neccessary, do some grey samples (with straight grey dyed yarn) and weave various interpretations of the pencil drawing in your three weekends.  That way you would solve the ribbing problem, experiment with the greys and interpretation without the added distraction of colour and complimentaries, which we could then concentrate on once you have finished the weekend workshops, both in paint and colour diagrams once the colour wheel exercise is done, as well as weaving and interpretation for Clip Horizontal. 
But it is your choice as to what you decide to do.