Following up on Douglas, we extracted grizY mags from the database in 
a small mag range around i=20 and picked out some fiducial points by eye
using the query in select_stars.sql.awk and awking the columns a bit.
We made color color plots of the dereddened color combinations and selected
some fiducial points by eye, those are in gri0locus, riz0locus,izY0locus.
you may wish to redo those. The Y-band is noisy because of our selection of stars at i=20, if you redo the select with say i between 17 and 17.05 the z-Y color
should look much better.

The columns of the stars.dat file:
Columns of stars.dat (0-indexed)
0 RA
1 DEC
2 g (PSF)
3 r (PSF)
4 i (PSF)
5 z (PSF)
6 Y (PSF)
7 E(B-V) SFD98
8 Spread_MODEL_I (< 0.002 indicates star or quasar, not extended object)
9 g0 using E(B-V) and 3.184 R_g coeff
10 r0 using E(B-V) and 2.130 R_r coeff
11 i0 using E(B-V) and 1.568 R_i coeff
12 z0 using E(B-V) and 1.196 R_z coeff
13 Y0 using E(B-V) and 1.048 R_Y coeff
14 (g-r)_0
15 (r-i)_0
16 (i-z)_0
17 (z-Y)_0
18 (g-r)
19 (r-i)
20 (i-z)
21 (z-Y)
22 magerr_g
23 magerr_r
24 magerr_i
25 magerr_z
26 magerr_Y
27 l Galactic longitude
28 b Galactic latitude