Is 10Gbps fiber (aqua or teal clad) backwards compatible with 4Gbps/8Gbps (normally orange clad fiber) FC equipment?

Just like the subject says.

I've got a mess of FC hardware in a rack that I'd like to replace the cabling on. Looking at Monoprice I see that LC/LC cables are really close in price between 10Gb and "normal" ($9.18 for a 1m 10Gb, $8.91 for a 1m "normal"). Is there any reason I shouldn't buy the aqua-colored stuff for future use?

Orange cable meets the OM2 standard. Aqua meets OM3 or OM4 standards (usually OM3). Yellow is singlemode cable.

The OM standards are backward compatible (for the most part; OM1 was defined to be 62.5/125 size; OM2+ are 50/125; but not all manufacturers followed this, just be sure to get the size that matches your equipment or you'll experience some attenuation).

No, go with the OM3/OM4. OM3/OM4 is becoming more common in the fiber industry. I manufacture fiber assemblies and see the trend moving toward OM3/OM4, particularly for future growth purposes. Within the 50um (micron) family, the Aqua fiber OM3/OM4 is backward compatible to its orange counterpart. Both fiber cores are the same size.