order fit can be applied to the whole interval:

$4000$ K $\leq~T_{\rm eff}~\leq~30000$ K (K7V - B0V)

($b-v$) $\in~[-0.30;1.30]$\
$V-v = -0.001 - 0.041 (b-v) - 0.035 (b-v)^2 \pm 2.10^{-3}$

### ($U-B$) vs ($u-b$)

$4250$ K $\leq~T_{\rm eff}~\leq~5500$ K (K6V - G8V)

($u-b$) $\in~[0.20;1.35]$\
$U-B =  0.120 + 1.087 (u-b) - 0.135 (u-b)^2 \pm 2.10^{-3}$

$5500$ K $\leq~T_{\rm eff}~\leq~10500$ K (G8V - B9V)

($u-b$) $\in~[-0.15;0.20]$\
No possible transformation.

$10500$ K $\leq~T-{\rm eff}~\leq~35000$ K (B9V - O8V)

($u-b$) $\in~[-1.4;-0.15]$\
$U-B =  0.003 + 0.819 (u-b) \pm 5.10^{-3}$

### ($U-B$) vs ($uvw1-b$)

$4000$ K $\leq~T_{\rm eff}~\leq~6500$ K (K7V - F5V)

($uvw1-b$) $\in~[0.20;3.20]$\
$U-B =  -0.292 + 0.710 (uvw1-b) - 0.051 (uvw1-b)^2 \pm 1.2\,10^{-2}$

$6500$ K $\leq~T_{\rm eff}~\leq~8750$ K (F5V - A3V)

($uvw1-b$) $\in~[0.10;0.20]$\
No possible transformation.

$8750$ K $\leq~T_{\rm eff}~\leq~35000$ K (A3V - O8V)

($uvw1-b$) $\in~[-2.20;0.10]$\
$U-B =  0.002 + 0.528 (uvw1-b) \pm 8.10^{-3}$

### ($B-V$) vs ($b-v$)

$3500$ K $\leq~T_{\rm eff}~\leq~35000$ K (M3V - O8V)

($b-v$) $\in~[-0.30;1.40]$\
$B-V =  0.002 + 0.997 (b-v) \pm 9.10^{-3}$

### ($U-V$) vs ($u-v$)

$4000$ K $\leq~T_{\rm eff}~\leq~6500$ K (K7V - F5V)

($u-v$) $\in~[0.40;2.80]$\
$U-V = 0.133 + 0.947 (u-v) \pm 2.3\, 10^{-2}$

$6500$ K $\leq~T_{\rm eff}~\leq~8500$ K (F5V - A4V)

($u-v$) $\in~[0.20;0.40]$\
No possible transformation, 