When is it desirable to miss color block or hit spikes?

I've only played Audiosurf 1, so I'm not sure how much this applies to 2, though I imagine the mechanics are largely unchanged.

Assuming all of the scores are legitimate, there are two things that could be getting people much higher scores than you: bigger matches and chaining.

Before I get into that there's one important concept you need to consider: the delay between a block landing in your grid and when the blocks finish matching and disappear. In Audiosurf 1, this delay was represented by a shape inside the colored blocks, which got smaller over time. Doing anything to your grid will reset this delay -- hitting another block (both colored and greys) and even removing greys.

The first thing, bigger matches, is pretty straightforward: the more colored blocks you have in your grid when they finish matching, the more points you get for it. Bigger matches are worth more than smaller matches combined, so a big match of 21 is worth much more than matches of 18 and 3.

It's worth mentioning that the delay period I mentioned will immediately finish if you try putting another block into a column that's already full.

The second thing is a bit more complicated. After you complete a match, there's a short window afterwards, where, if you continue hitting blocks and keeping your "delay" going, you'll get bonus points. In Audiosurf 1, this was represented by a small horizontal bar on the bottom of your screen getting progressively bigger, but I don't know what it looks like in Audiosurf 2.

If you keep this chain alive as long as possible, you will get more and more points! Again, like with the delay, doing anything to your grid will keep the chain alive, even if you don't currently have any matches going.

It might be more difficult to do in mono mode, but I know in the other modes a common tactic was to try and get many small matches at the beginning of a song to quickly build up the chain.

On the other hand, if Audiosurf 2 still has the massive bonus at the end for not hitting a single grey, that could outweigh the benefit you get from more chaining.

Make sure you're only weighing your score against other mono players. Mono just isn't good on certain types of songs!