These guys sell a wide range of coloured silicone sealants.
I would however recommend using clear silicone for the 90 degree return. Glass with the back painted has some rather odd optical qualities. Even if you had a perfect colour match silicone I am worried that the join would stick out like a sore thumb, erm, one that is clearly sitting on the surface. The clear silicone will create a subtle shadow line.
If your glass was coloured all the through, then yes I would recommend a colour matched silicone.
The clear silicone will additionally make it much easier to seal the join between the glass to the worktop. Where the glass meets the plasterwork- do not use silicone- it will create problems when you try to repaint the walls. Standard caulk or clear CT1 will be fine.