diff --git a/src/devices/bus/interpro/sr/edge.cpp b/src/devices/bus/interpro/sr/edge.cpp
index 1228450375a..4c6453a5341 100644
--- a/src/devices/bus/interpro/sr/edge.cpp
+++ b/src/devices/bus/interpro/sr/edge.cpp
@@ -755,9 +755,9 @@ u32 edge2plus_framebuffer_device_base::screen_update(screen_device &screen, bitm
 			const u8 index = *pixel_data++;
 
 			bitmap.pix(y, x) = rgb_t(
-				m_ramdac[0]->palette_lookup(index),
-				m_ramdac[1]->palette_lookup(index),
-				m_ramdac[2]->palette_lookup(index));
+				m_ramdac[0]->lookup(index),
+				m_ramdac[1]->lookup(index),
+				m_ramdac[2]->lookup(index));
 		}
 
 	return 0;
diff --git a/src/devices/video/bt45x.cpp b/src/devices/video/bt45x.cpp
index 4ab4f687e67..acf60ce238d 100644
--- a/src/devices/video/bt45x.cpp
+++ b/src/devices/video/bt45x.cpp
@@ -29,7 +29,7 @@
  *
  * TODO
  *   - refactor to separate devices with registers
- *   - implement blinking and overlay
+ *   - implement blinking and rgb device overlay
  *   - unsure about address masking when accessing overlay colors
  */
 
diff --git a/src/devices/video/bt45x.h b/src/devices/video/bt45x.h
index df1114f74dd..f6d3a3144ab 100644
--- a/src/devices/video/bt45x.h
+++ b/src/devices/video/bt45x.h
@@ -97,9 +97,14 @@ protected:
 class bt45x_mono_device_base : public bt45x_device_base
 {
 public:
-	// helpers instead of a device_palette_interface
-	u8 palette_lookup(u8 index) const { return m_color_ram[index & m_read_mask]; }
-	u8 overlay_lookup(u8 index) const { return m_color_ram[m_palette_colors + index]; }
+	// helper instead of device_palette_interface
+	u8 lookup(u8 pixel, u8 overlay = 0) const
+	{
+		if (overlay & 3)
+			return m_color_ram[m_palette_colors + (overlay & (m_command & (CR1|CR0)))];
+		else
+			return (m_command & CR6) ? m_color_ram[pixel & m_read_mask] : m_color_ram[m_palette_colors + 0];
+	}
 
 protected:
 	bt45x_mono_device_base(const machine_config &mconfig, device_type type, const char *tag, device_t *owner, uint32_t clock, const u32 palette_colors, const u32 overlay_colors);
diff --git a/src/mame/drivers/ncd68k.cpp b/src/mame/drivers/ncd68k.cpp
index e1a4e99e3fb..7e458e3e566 100644
--- a/src/mame/drivers/ncd68k.cpp
+++ b/src/mame/drivers/ncd68k.cpp
@@ -231,7 +231,7 @@ u32 ncd17c_state::screen_update(screen_device &screen, bitmap_rgb32 &bitmap, rec
 		for (unsigned x = 0; x < 1024; x++)
 		{
 			u8 const pixels = m_vram->read((y * 1024) + BYTE4_XOR_BE(x));
-			*scanline++ = m_ramdac->palette_lookup(pixels);
+			*scanline++ = m_ramdac->lookup(pixels);
 		}
 	}