[cheese (MSW)]

(a) Why is Si2(Mes)4 yellow?
(b) Why is Si-29 NMR spectroscopy not used more routinely?

[Mitch]

(b) has to do in part with the low isotopic abundance of Si-29 (~5%).

[fledarmus]

Another issue is that the silicon in the NMR tube and in the probe give a large background signal that has to be removed.

a summary here:

http://chem.ch.huji.ac.il/nmr/techniques/1d/row3/si.html

[cheese (MSW)]

(a) Mes2C=CMes2 is I believe colorless.  Mes = mesityl: -C6H2(2,4,6-Me)3

(b)  Receptivity = a × γ^3
a = natural abundance; γ = gyromagnetic  ratio
When you plug the numbers in for C-13 and Si-29, the Si-29 nucleus (spin ½) is about twice as receptive as C-13.  And of course C-13 NMR spectroscopy is routine.
A typical NMR tube has a broad peak from -80 to -300 ppm.  This does not usually affect the spectra of organo-Si cmpds that have shifts in the range +100 to -100 ppm.  Of interest is the reference is TMS (SiMe4) that as you know is δ = 0.0 for H-1 and C-13.  (But my compliments for thinking outside the box!)

The answers to these questions have considerable pedagogical value so thank you for your posts.

[cheese (MSW)]

More hints:  (a)  π → π* and overlaps
(b) The gyriomagnetic ratio of Si-29 has the opposite sign to H-1!