Great to see your diagram - thanks for sharing. You've done a great job of getting the concept across.
Notice that I also uploaded a png version so that others can easily view in their browsers. It is always a great idea to upload both the vector (svg) and raster (png) versions.
A few comments for you:
1) You mention wanting to make the tree a different color. This can be achieved with the online diagrammer by clicking the "Break Link to Reference Element" button (chain link), then ungroup. This will allow you to select the various components of a symbol and edit any aspect of it.
2) In your other post
you ask about filling an area. Now that I can see your diagram, I have a better idea of your needs. In a vector shape you need to create a continuous polygon to be able to fill it, so the background area of your phloem and bark sections would need to be a proper polygon, rather than separate lines on each edge. Vector is a very different approach to drawing, but once you get the hang of it, you won't ever want to go back to raster because of the ease and precision of editing and the scalability.
3) I see that you mostly used the pencil tool. I would recommend that in most case the Path Tool is superior - with it you can eliminate the roughness of the lines by reducing the number of nodes (vertices) of each path and using the control handles (click the path twice to bring up these editing handles) to accurately define the shape of the path.